Some users photo's contain porn or inappropriate content. Some of these albums have been un-noticed for months by facebook staff and even when they're reported, nothing seems to get done.
In many cases, these can also be on fan pages and groups which anyone (even children) are able to find and access.
It's okay and good to use to communicate and to socailize with other people. But, you have to be careful of, who you invite and what links you click on because of spammers, hidden exploits, and viruses and worms like Koobface. Also, some apps are malicious so watch for those, too. By the way, what's on Facebook stays on Facebook. Which is why you should be cautious about not revealing too much private information.
Depending on who you have in your friends list, this site can either be extremely pleasant or extremely hateful and racist.
I've personally not experienced anything hateful but I know people who have, there is little censorship on the site so be warned.