Google Inc. (GOOG)’s new social-networking service, an effort to lure away users from Facebook Inc., saw U.S. visits soar 13-fold last week after the site opened to the general public, Experian Hitwise said in a report.The Google+ site received almost 15 million U.S. visits in the week ended Sept. 24, compared with 1.1 million the previous week, according to Hitwise, a research firm in New York. Until last week, the service was in an early test phase, and users could only join if they were invited.Google, the world’s biggest Internet-search company, is trying to leverage that dominance to gain more social-networking users. When it opened Google+ to the public last week, it also added new search capabilities, making it easier to find posts from friends and other content.With the changes, Google boosted the number of Google+ features to 100. The company is trying to provide reasons to leave Facebook, which has more than 750 million members. Google also unveiled an online-game service last month with developers such as Zynga Inc., maker of the Facebook game “FarmVille,” and Rovio Entertainment Oy, creator of the hit “Angry Birds.”Google rose $6.38, or 1.2 percent, to $531.89 today in Nasdaq Stock Market trading. Shares of the Mountain View, California-based company have declined 10 percent this year.