Personally I reckon you should focus on creating a truly awesome FB client.
There's already way more Maemo 6x Twitter clients than there are FB ones.
There's that good S^3 client (name escapes me) that does multiple social networks.
But it's Twitter utility/usability is now falling behind some of the best Twitter clients.
You spread yourself thinly & you run the risk of not being very good, just mediocre at more things.

Maybe longer-term you could integrate some sort of compatibly with Diaspora?
It's much more like FB than Twitter, just a more "FOSS friendly" version...
Google+ client may be a good idea too, but IMO the hard focus must remain on FB.
Unless you feel you can branch out & still maintain very good utility/usability for everything.