Web-traffic watcher comScore Inc. estimated Google+ has had 20 million unique visitors since its launch, including five million visitors from the U.S. A Google spokeswoman declined comment. ComScore, whose estimates are based on a “global measurement panel” of two million Internet users, similar to the approach Nielsen uses to measure television ratings,doesn’t have data on the number of minutes people spent on Google+. Still, the growth of Google+ has impressed observers because access to it is by invitation only, meaning people can join only if a member invites them. And the company hasn’t yet marketed the service to the more than one billion monthly visitors who use its search engine, Gmail and other services.