Your friend Monica calls you, agitated and angry, asking: "Why did you write that horrible thing about Ross?!"

You never said that, but all your friends on Facebook think you did because a profile claiming to be you has said it.

At a glance the account could be yours too.

From photos, to status updates, to your date of birth, the account is rich with information which has been lifted from your profile, which is public.

It is so convincing many of your friends have accepted a friend request.

The problem is that you have no control over what the account pretending to be you says, or who it says it to.

After being granted rare access to Devon and Cornwall Police's Hi-Tech Crime Unit, Newsbeat has learnt this scenario is becoming more frequent, particularly for young people.

It's thought weak privacy settings combined with a rise in professional trolls who have the time and resources to steal your identity is to blame.

An anti-bullying group has told us replica accounts have sent death threats or told people "to get cancer".

There have been incidents where people have been beaten up or were scared to leave their house after offensive messages were sent from an account pretending to be them.

Replicating a profile is all about piecing bits of information together, like a jigsaw. The more pieces there are, the more believable the account will be.

While you can never completely stop a troll from stealing your identity, here are some things you can do which will make a troll's life a bit more difficult.

Facebook

The best way to protect your profile is to make it completely private so only people you know can see it.

However, even if your account has the highest privacy settings, if a couple of your friends have Public accounts and accept Friend Requests from strangers then a potential troll could still access your photos or comments.

By editing your account so only your Friends can see your Friend List it will be harder for a troll to work out who you know.

If you want to see how strangers view your Facebook profile click on View As (see above) and it will show you.

We think Rachel may want to reconsider how much she is sharing with the world and keep it amongst her "Friends"...

Sorry.

Instagram

There's nothing wrong with letting people know where you are when you have taken a photo; if you are capturing the Eiffel Tower it can feel almost essential.

However, photos can soon add up, and if you do not keep an eye on your Instagram Photo Map and it is open to the public it soon becomes clear where you live, work and socialise.

That's handy for a troll.

You can edit what pictures appear on your map by clicking on them.

If you don't want any locations on any of your images you can make the switch on your privacy settings.

Twitter

It's one thing tweeting that you are rocking out to Taylor Swift at Radio 1's Big Weekend 2015 and the message confirms to the world you are in Norwich.

It's a bit different if you are at home and decide to tweet about Rita Ora on The Voice and the message confirms to the world exactly where you live.

To remove your location from a tweet choose Security and Privacy on the Account tab and untick Tweet Location.

You may need to re-enter your password, but once you have done that you can tweet away from the comfort of your home, happy in the knowledge that only the people you want to know where you live, do.

Snapchat

The beauty of Snapchat is you take photos which you feel are important, for ten seconds or so, and then they disappear.

But are you sending these pictures to your Friends or to Everyone...

If your Privacy Settings looks like this, you can change it so only your Friends can see your photos.

It's also important to be careful who is on your friends list to make sure you are only sharing your photos with those that you want to.

Detective Constable David Wright from Devon and Cornwall's Hi-Tech Crime Unit told Newsbeat: "There are people with time and resources who have embarked on almost a career in creating an online presence pretending to be someone else.

"I don't want to be too sinister because social media is supposed to be fun, but sometimes we do need to think about what we are sharing online."

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