Rob. Great points. In making our point, we focused on the aging of the Facebook population, which is a caused by a combination of older demographics joining and younger demographics leaving. So, yes, Facebook's population may be matching the US. That's part of the point.
As Facebook becomes a site for "everyone", it potentially becomes less attractive for people who want a site that matches their interest and style (and those sites will skew younger than the US population). We can go longer with the data. I'm not doubting that the trends will be similar to what we highlighted (but that's a another infographic). I appreciate your comment!