This very clearly demonstrates Facebook's advantage: despite recent buzz about Facebook being boring, losing its vision, etc, they have built an enormous moat through the developer platform, and the true test for the competition (Google+ or otherwise) is whether or not they can compete on that platform level.

Actual report:

http://www.rhsmith.umd.edu/digits/pdfs_docs/research/2011/Ap...

Not sure I buy (or fully understand) their estimates - there seem to be a lot of unstated assumptions in there.