"Is it possible to do jihad half a year?" asks one would-be recruit, apparently seeking to join a rebel group in Syria on a short-stay basis. The question is directed at a man on the Ask.fm website who describes himself as "a stranger currently fighting in Syria".

Other questioners seek advice on how easy it is to cross the border into Syria from Turkey and the best way to persuade parents to allow them to go.

They also want to know: "How much is a Glock pistol?" "Which is the best rebel group to join?" "Is there training in jujitsu?" One of the most extraordinary questions, given the carnage in Syria, is: "Would it be wise to come there with my wife and one-year-old son?" The man replies, not surprisingly: "The situation right now isn't the best for families."

There are similar question-and-answer sessions, chats, pictures and videos on Facebook, Kik, Waptrick and a host of other social websites, with Facebook the most popular. British youngsters post pictures of themselves in the desert or in hotel courtyards in Syria, in shades, brandishing Kalashnikovs and inviting friends to join them.

Many of these Facebook jihadis will return to Britain and pick up their old lives. Others, moved by what they have seen in Syria, will embrace charity projects. What alarms security agencies is a tiny number who, having coming into contact with anti-western al-Qaida-linked groups, might come back plotting terror.

The security services have been tracking Britons travelling to Syria for several years and, with concern growing, are now devoting more resources to this. There have been only a handful of arrests so far; the most serious relates to an alleged plot, while others are for allegedly attending training camps in Syria.

The British government, like other European nations, is struggling with how to respond. Since April it has been taking pre-emptive action, blocking travel for those suspected by the security agencies of heading for training or fighting in Syria. The home secretary has used the royal prerogative 14 times to refuse applications for passports or confiscate them.

The security services are preparing for a long haul. The home office minister responsible for security, James Brokenshire, giving evidence to a home affairs select committee in March, said: "I do see this as an issue that will be with us for the foreseeable future, because of the numbers that have travelled and the risk attaching to individuals returning."

During the Iraq conflict, the number returning was proportionally small, partly because of the intensity of the fight: there were no insurgent-held safe areas, and new arrivals were drafted in as suicide bombers or became the victims of relentless tracking by US forces.

But those travelling to Syria have flexibility: they are able to choose how long they go for, in some cases just a matter of weeks; and they can decide the level of risk they are exposed to, by opting for a hotel in one of the rebel-held areas well back from the fighting, with maybe a fleeting visit to the frontline, or joining a hardline group that regularly confronts the forces of Bashar al-Assad.

Fighters of the al-Qaida-linked Islamic State in Iraq and the Levant carry their weapons during a parade in Syria, near the border with Turkey. British youngsters are among recruits to such groups, prompting fears that they will return to the UK as potential terrorists. Photograph: Reuters

The would-be jihadis can fly to Turkey and make their way to the border, where they can volunteer for one of a variety of rebel groups or one of the humanitarian convoys. They are mainly male, aged between 22 and 40. An estimated 400 have gone so far from the UK, of whom about half have returned.

On the Ask.fm question and answer session, the "stranger currently fighting in Syria" advises that it is still possible to cross the border from Turkey "but tougher than earlier". He deals, too, with a proposal of marriage, saying he is not looking for a wife. Asked about whether anyone from the al-Qaida-linked rebel group Isis (Islamic State in Iraq and the Levant) might be looking for a wife, he says: "I would sincerely advice [sic] you to stay away from Isis."

Had he come across any suspected spies? "Is Syria the den of spies we have been led to believe?" He replies: "No, I haven't. To suspect someone without evidence is haram [sinful]."

One of the key questions is his motivation. "What keeps you motivated to fight and go on?" Referring to barrel bombs widely used in Syria, he replies: "When you see a little girl killed by barrels or an old man crying cuz his house was bombed."

Raffaello Pantucci, author of a history of jihadism in the UK, We Love Death As You Love Life: Britain's Suburban Mujahideen, said such scenes of carnage could lead to blowback in the UK. Although Britain is not directly involved in Syria as it was in Iraq, Pantucci said that returning jihadis might contrast people wandering around the streets of Britain eating ice-cream with a remembered scene of a child with its head blown off in Aleppo. "There is not necessarily a logical connection," he said.

Pantucci said the social media component of the conflict was relevant mainly because of its immediacy. "On Kik, on Ask.fm, people ask questions and you see people saying 'I want to come and how can I get there?'" he said.

Another counter-terrorism specialist, Thomas Hegghammer, regards social media as the source of a lot of recruitment. "I suspect a lot of people are inspired to go by seeing their friends go and seeing them with weapons on their Facebook feed and building up the motivation to go themselves," he said. "There is definitely a glamour element to it, a chance to show off, as if the picture with weapons is the main objective. For many of the recruits, it is attention-seeking, to get the respect of peers. And they do. You can measure it in the comments people leave on the pictures and the number of 'likes' they accumulate."

Unlike earlier conflicts when static web forums used by jihadis could be taken down, social media is relatively risk-free and harder for governments to close down or mount prosecutions against.

Hegghammer, who is director of terrorism research at the Norwegian Defence Research Establishment, has estimated that about one in nine of those returning might engage in a terrorist plot. But he was lumping together Syria, Afghanistan and other conflicts. His estimate for Syria alone is lower. "You do not get the same blowback from Syria. It could be as low as one in a hundred. If I was to guess, it would be one out of 20 to 30 over time, over a 10-year period," he said. But even a few is enough to worry the security services.

When the Syrian conflict began, there was just a trickle of volunteers, then numbers began to pick up in 2012. Between May and September last year, the numbers grew quickly, with some European contingents doubling. Since then, the pace of growth has slowed. The number who have left Europe for Syria is estimated at 2,000.

In the US, addressing the Washington Institute in March, Shaarik Zafar, deputy chief of the homeland, cyber and countering violent extremism group, put the number going to Syria from round the world at 7,000, part of a 110,000-strong force fighting Assad's troops. They had come from 40 countries in Europe, the Middle East, Africa, central Asia and Australia, with some going to the al-Qaida-linked groups and others to secular ones.

Zafar said there was no single motivation. Some were simply against the Assad regime; others were driven by humanitarian reasons or a sense of adventure or as a result of social media. That is where young people are, so that is why the terrorists are there, he said.