"Overtake" is an ambiguous word. If these extrapolations are correct then Facebook will get more unique visitors than Google some time in the next year or two,

"Overtake" is an ambiguous word. If these extrapolations are correct then Facebook will get more unique visitors than Google some time in the next year or two, but why use this metric rather than (e.g.) total number of visits, total amount of traffic, amount of time spent at the site, revenue, or profit? On some of these I would guess that Facebook's been ahead for a while; on some, Google is probably well ahead for the foreseeable future. In any case, comparing any metric of this sort between two sites that are so different, and that do such different things with their visitors, seems rather pointless to me. Like comparing unit sales volumes between a company that makes mobile phones and one that makes cuddly toys, or comparing one hospital's survival rates for heart transplants with another's for cancer surgery.