I've read that hundreds of Facebook groups have been hijacked. At first, Facebook denied it, but it's true. To read about this, click here.
Now, attackers have released a newer, more powerful version of the Koobface virus. (Koobface is an anagram of Facebook.) More than 300 Facebook groups have been taken over in the last few days. From what I've read, it infects a PC, then sends messages and wall postings to that computer owner's friends. If the friends follow the links, they can end up infected, too.
To protect yourself, do the stuff you've already heard you should do. Keep your virus protection up to date. If a link looks suspicious, even if from someone you know, don't follow it. If you find yourself on a page that is asking you to download a software update, don't do it.
You can read more about it here.