Remember 2009's Obsessed? It tells the tale of a man whose life is torn apart by a temp worker who falls in love with him and will stop at nothing to win him over. The plot is (hopefully) exaggerated for thrill effect, but the mentally deranged woman showcases some serious Internet sleuthing and stalking. It's certainly not a common tale, but serves as an unlikely reminder that some things are best kept private.
While most social networks offer enhanced privacy settings, which will allow you to keep your profiles private, there are a handful of things you can do to make your accounts less accessible and susceptible to searches made by your new guy's ex or an old friend you don't want to have to ignore online.