One big difference I have found between Facebook games (of which I play a few) and mobile games on my iPad (again, I play a few, something had to fill the gaping hole in my free time that used to be filled with CoH) is that many Facebook games require Flash, which I don't have on my tablet. Therefore, I can read Facebook on the iPad, but I can't play the games. If this specific piece of saftware wasn't required, I would happily play my Facebook games on my mobile device.
I spend a bit of money on FB games, picking up a bit of in-game currency, since I have an extra $15 sitting around unused every month, but I would never drop more than that on them, so losing what I spent on a Facebook game is no big deal to me, but I guess YMMV.