The Facebook App Center is definitely interesting. They've announced you can make paid apps as well, where you charge a flat fee to gain access to the app (like other app stores) rather than using IAP exclusively. It could be an interesting new monetisation strategy. I'll be investigating it myself soon, but if anyone has a Facebook game they're adapting for App Center, please let us know how it goes for you!
I'd love to add support for Google+ games as well, but AFAIK they still haven't opened it up to third party developers. Their developers page does not mention games at all. So as long as they are only developing privately, we're locked out from supporting it. I think Google do want to eventually allow third party games, it's probably just taking a long time to set up.