Ah yes, all those trendy young game players with their new fangled Facebook accounts. But seriously, isn't this just the same argument that was made for 'Crypto Gravity' and it's potential on the iOS Store and the Play Store? How's that panning out? I'm reasonably convinced that the lack of popularity is simply due to it being a bit of a poor product, rather than it needing to be on yet more platforms.
If mobile gaming has a somewhat... uhmmm... "casual" character then Facebook gaming even more so. Pretty much the opposite of niche retro platformer audience Gravity appears to cater for. Doesn't seem to be any rationale behind this, other than "oooh, Unity supports Facebook, millions of potential bagholders, moon".