getting facebook to sync is tricky. The "can't fail" method I use is this :
1) go into People
2) tap on the little person icon with the sync symbol on it.
3) press menu
4) tap refresh
5) do steps 3 and 4 in rapid succession 4 or 5 times.
6) Leave the phone ALONE on this screen Let the screen sleep. Don't touch the phone for at least 10 minutes. All of your contacts should be synced. If they aren't do all those steps again.
I have 400 some odd facebook contacts and this syncs them for me.
Once synced they should stay that way, but sometimes new contacts don't so just do it again.