in reply to
How to prevent impersonation of other users
Just a quick note in concurrence with the post of Yendor, with whom, in principle, I voice no disagreement: there is also the possibility (however remote) that someone may have the exact 'screen name' as your friend, but be someone else entirely. (creative people can think of numerous ways that could happen)
Thus it is just another situation to be wary of, and another reason why healthy intellectual skepticism will always be a useful tool to have in one's internet-navigational toolbox.