To quote Mad Eye Moody, "Constant Vigilance!" is now a necessary requirement for avid Facebook users.

A
slew of worms and viruses are working their way through the social
networking Web site, attacking profiles, spamming inboxes and adding
commercial links to groups and personal content. A particularly vicious
strain, the Koobface virus, which became a problem in December 2008 and
has now morphed into a more powerful form, actually hijacks the user's computer system and "turns it into a zombie."

The 2.0 version of the Koobface virus is sly and manipulative beyond the realms of its malignant
predecessor. Posing as a link from a friend, the virus looks like the
page of a Youtube video featuring a selected user's "friend." Once
clicked, the user will be asked to update his or her video software by
downloading what turns out to be the virus in disguise. Technology
websites are warning Facebookers that the new viral threats look
deceptively benign.

The mutated form of the Koobface virus
will even display the user's name and photo on its fake Youtube page.
Once the false "update" has been downloaded, the infected user's
Facebook profile becomes a platform entirely subject to the will of the
virus lords, who will change group names and send spam emails to the
infected user's friends.

Facebook is going to be blotchy at least for today as developers try to
figure out how to kill this pesky worm, which has quickly become a
vigorous nemesis.

Advice on how to avoid the worm is unanimous:
avoid clicking on unnecessary links, joining obscure groups, accepting
update requests and ... yes ... making friends with people you don't
know.

At least for now, safe Facebook behavior means conservatism. Keep your friends close, so that you're enemies can't get closer.