SAN FRANCISCO — Since Facebook started giving out customized Web addresses like facebook.com/yourname last Friday, some 9.5 million people have rushed to grab their top choice.

On Twitter, public fights have broken out over so-called impostor accounts, like those that should probably be in the hands of Kanye West or Bank of America. And somewhere out there on the Web, another new service or social network is on the rise, threatening to start yet another online land grab...