Some people think it might be. Twitter users started using hashtags - a # followed by a unique bit of text - to help them follow conversations. Through some eerie demonstration of the hive mind, Twitter users usually come to a consensus on the tag or tags for an event or topic.

As with email and blog trackbacks, anything that becomes successful on the web ultimately becomes the target of spammers. Users began to notice a few months ago that tweets on popular hashtags often contained marketing messages or links to porn.

Upscale furniture maker Habitat was caught out last month when marketers working for the company used hashtags, even ones related to the disputed Iranian election, to promote its "totally desirable Spring collection". Twitter users cried foul, and Habitat apologised.

That hasn't stopped less scrupulous companies from continuing to use hashtags for spam. Hashtags around the death of Michael Jackson often have unrelated commercial messages. Beyond spam, the internet pranksters of 4Chan recently "attacked" Twitter (in the words of some web watchers) by creating multiple accounts and pushing the hashtag #gorillapenis into the trending topics.

This has led many to ask if the hashtag system is too prone to abuse. Twitter's openness has been praised as one of the key reasons behind its meteoric growth. But is it that openness also an achilles heel?

Social media blog Mashable wrote: "Trending topics are a great way to find out what's hot in the Twitterverse, but they're also a haven for malicious hackers and spammers."

Of course, not everything slightly off-colour is spam. Last week the hashtag #MrsSlocombesPussy trended (at the urging of Jonathan Ross) after the death of actress Mollie Sugden. British Twitter users and American fans of Are You Being Served? spotted it at once as a tribute to the actor; others, such as TechCrunch and Mashable thought it was just more pornographic spam, until set straight by their readers. Some accused Twitter of censoring the hashtag, but Twitter co-founder Biz Stone told appscout.com: "We don't filter out offensive content from search. There's a bug involving hashtagged words with more than 16 characters." (MrsSlocombesPussy has 17.)

But some companies think that Twitter should capitalise on their eagerness to use the sevice to promote their businesses. Moonfruit, a website building service, has been giving away MacBook Pro laptops in return for tweets with its hashtag. It trended - but then disappeared. The company wondered whether Twitter was suppressing it, but was unable to get any confirmation.

Moonfruit's Wendy Tan White thinks that Twitter is missing a trick. "This is probably a commercial channel for Twitter in the future."