Google+ is pretty rad. It has a Twitter element, in that you can follow others and have others follow you without being linked together as friends, and it hasn't been around long enough to pick up some of Facebook's bad habits, like bungled policy controls. But for all of its potential, Facebook is still the No. 1 social site on the planet and it's showing no signs of slowing down.
Facebook doesn't publish unique visitor data by country, but comScore does. According to comScore (PDF) , Facebook attracted over 162 million U.S. visitors during the month of July 2011, which is the highest it's ever been. Prior to that, the record was 160.9 million, recorded one month prior.
It will be interesting to keep an eye on the number of unique visitors as Google+ gains steam. Google+ launched at the end of June, but only to a limited number of users. Invites were few and far between at the time, and still today Google+ is by invite-only.