Just regular above quality photos for Facebook and normal things.
Facebook images are usually displayed at around 720×540 pixels. That is about one third of a megapixel, so for that use, anything you can buy is overkill in terms of megapixels. This is perfectly fine for almost all online display, and will even make decent small-sized prints.
However, megapixels do not accurately represent image quality. The answers to Why do some DSLRs have fewer megapixels than some pocket cameras? provide some of the explanation as to why. You can't really use that as a way to make your decision.
Looking at the smartphone market I would say any phone with 5Mpx can be a good choice. (For example the old Xperia X10 mini does pretty good photos for its price). But smartphone cameras are always a huge compromise work and as said Mpx never tell so much of the story. So maybe it is better to chose two or three models and compare them here http://www.gsmarena.com/piccmp.php3