Yes some are Micro Gaming I play them too and Jackpot Party... and I think the reason is and if I'm wrong I hope someone can correct me. In Europe it's legal to gamble on Facebook.
So the same games are on USA side just the payout/ play for fun only.
I think Facebook get around gambling laws by treating it as an online video game, much like World of Warcraft, where you can put money in, but can't get MONEY out. The loophole seems to be that they now offer "real world" prizes, although not money, so it's not seen as gambling.
It's as good as gambling though, because "real world" goods can be exchanged for money by selling them on eBay.
They also award free credits on a regular basis, and these too can be used to win the same prizes as those who pay, which seems another route around the gambling laws. It could present a FANTASTIC opportunity for Advantage Players, as with patience and discipline, it should be possible to win loads of prizes without parting with a cent. It's then just a question of turning the prizes into cash, for which we have eBay.
The downside is that you are sometimes bombarded with ads, often to buy extra credits which are on permanent "special offer".
I have been looking into "Clickfun" since last year, and have reached level 113 on free coins alone. They are actually using a social version of Microgaming, so if available in the US, it's a means for US players to reconnect with Microgaming slots. However, they are not the same games, they work on a different principle, and have different features and paytables.
Clickfun have recently started to run tournaments, but I am sure this is to spur participants into buying extra credits in order to achieve a winning position.
Once you start winning real prizes, you will have to give the app your address details, which is often more info than on your profile (which the app has access to anyway).
The explosion in the number of "free gambling" apps is what presents a potential opportunity for advantage players. It may also have the side effect of reducing the burden advantage players are to real money online casinos.