Are you on Google+ yet? Chances are yes: Google’s new social-networking site could hit 10 million users by the end of the day Tuesday, at least according to one estimate. Ancestry.com founder Paul Allen calculated the network’s size and found that the social network has grown 350 percent in the past six days—and could reach 20 million users by the weekend. By Allen’s own admission, the methodology isn’t perfect; he calculated by using U.S. surnames and then extrapolating from a ratio from U.S. to non-U.S. users. New users join Google+ by invitation, so it’s likely to keep growing at the same rate or faster.