So here's some attempt at analysis. This is all nonsense, of course; just me playing with numbers. Also, it'll make somebody angry somewhere, because it always does, but I can't help that.
Like I mentioned, I've used official company accounts only. I haven't included Monte Cook's personal twitter feed, or Mike Mearls' or the many fan communities on Facebook or G+. This is only a measure of how companies are using social media, not how popular their games are.
Also, yes, I know other social media exists, including Reddit etc. Anyone is welcome to adjust these figures by adding other platforms in.
Total social media usage of those 7 companies for those three networks is as follows. WotC's Facebook investment skews this massively; it's also interesting to see that companies aren't using G+ much, although fans certainly are - there are some enormous unofficial G+ communities for lots of games.
Looking at the total amount of social media used by those seven companies, irrespective of platform, we get the following view. Again, this is skewed massively by WotC.
Of course, there are likely others I haven't covered. This is just a casual impression, not a scientific study. Feel free to use these figures, correct them, use them, ignore them, whatever.