Given Facebook's near-ubiquity and its habit of changing user communication preferences on a whim, it's no surprise that it's the frequent subject of fake message scams from criminals. One scam email currently doing the rounds (I've seen it at least a dozen times in my own account over the last week) pretends to contain notifications of new friend requests in order to con unsuspecting users into clicking on a link.
As Sophos' Naked Security blog notes, right now the link actually redirects to an online pharmacy selling Viagra, but that could easily be changed to a more malicious site. If you've got decent spam filtering options, chances are you won't see these emails, but remember: it's rarely a wise idea to click links in emails, even if they look authentic. (As you can see in the screenshot, a quick hover over the link shows this one isn't.)
While social networks themselves are increasing used for scamming, email is still a popular medium. For more tips, check out our top 10 ways to stay secure on social networks.
Beware fake Facebook notifications arriving in your email [Naked Security]