Great site to keep in touch with friends and family, though I would stay away from the facebook games if you don't want viruses, as some of them open your computer up to infection. Those are third party programs though. Facebook itself is safe. Just stay away from the third party applications and you should be fine.
Facebook itself is a good website and has above average security. Flaws come in the form of Apps and ignorant users. Facebook is not for children who are not closely monitored by their parents. Set your password strong (not something in the dictionary or your favorite pets name); don't allow Apps access to your profile; don't post anything you don't want pubic to anyone; don't set default settings lower than Friends Only. "Friends of Friends" is not safe.
Take this from a network penetration tester -- most of the ways we gain access to a company is through social networking by their employees. You'd be surprised how much everyone feels they need to talk about private stuff.
Facebook has been messing around with privacy of anything that users put up. Plus, they've been putting apps up instead of application about pages everywhere, which makes it more difficult to block stuff like FarmVille, Mafia Wars, etc. No, I won't play any of that garbage!