Pros: Good for social networking, finding old friends.
Cons: Very poor respect for privacy. Very shady in their implementation of site changes, the majority of which are not for the benefit of the members. The terms of service and privacy settings are changed every few weeks, usually to force members into sharing more and more data. You must be vigilant of your settings on the site and very careful about what you post.
All in all, Facebook is a quality website, allowing you to keep up with your friends, though there is one big flaw...
The Applications API gives developers full access to your accounts, such as your personal details, and all of your friends. This could lead to email addresses being harvested and sold to spammers.