What is it about facebook photo sharing that makes it best for you? Is it that the photos of most interest to you are there? Is it a certain point of integration with another facebook feature? Is there a unique feature of their photo sharing function that you like?
1.) They have a pretty good multi-file uploader, so you can take a whole directory of files and upload them at once.
2.) It's free. Many of the other sites with #1 require that you pay for it.
3.) You can tag your friends, and a notification in their news feed will pop up saying they've been tagged in a photo. And all of their friends will see it, usually leading to tags for people you don't know.
4.) Enough of my friends are on it that #3 is useful.
A lot of this is network effects - I think Flickr has all of the above, though some may be premium features. But FaceBook has a critical mass of people that check it regularly, which makes 3 & 4 a lot more useful. If I post to Flickr and tag my friends (can I do that?), there's no guarantee they'll see it.
The tagging makes it the best. The whole low-resolution thing makes artistic photos not work out too well, but for pictures containing people, Facebook really is the best. Especially if you're in college, in which case nearly everyone has a fb account.
