So how many people are using Google+? Google’s not saying, but it has opened invitations up again. There’s no shortage of speculation. Ancestry.com’s founder, based on the occurrence of surnames in the U.S., estimates Google+ might have 4.7 million users. A search string produces a figure of 9 million. Bill Gross says Google will get to 100 million faster than anything before it because it’s, well, “great,” and Google is “huge” (Twitter and Facebook weren’t, when they started). Recall that Google Buzz started fast, too, before puttering out. Some professional users are irked by its terms of service (photo licensing, in this case) but I think Google could make a good case for developers. If only it would hurry up with an API and ad network integration story.