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The news, predicted earlier this week and noted a few minutes ago on Cecilia Kang's Post Tech blog ... here, suggests that neither worries about privacy nor other complaints raised about the social-networking site have slowed its growth in any meaningful way.

To put this rate of growth in perspective, a year and a half ago, it was a big deal for Facebook to hit the 150-million-user mark. By last Sept. 15, it had more than doubled in size to 350 million users. February saw it announce its 400 millionth user.

Now it can claim about 7.3 percent of the world's population -- though we can't tell how many of those 500 million accounts are duplicates set up by people to promote their work, cheat on significant others or hide an espionage assignment.

To commemorate the occasion, the Palo Alto, Calif., company has set up a "Thanks" page featuring photos of its employees expressing their gratitude, plus a new application called "Facebook Stories," in which users can testify to their experience of the site. You can browse by location (for example, a Kensington resident tells how Facebook helped him adopt his child) or search by theme. Your choices in the latter include "Crime Fighting," "Reunions," "Lost and Found," and "Music" ... but not "Drunk Facebooking" or "Shouldn't Have Friended My Boss."

(There-will-be-a-quiz-on-this-afterwards disclaimers: Post Co. Chairman and Chief Executive Donald E. Graham sits on Facebook's board of directors, while the newspaper and an increasing number of Post staffers, myself included, use Facebook for marketing purposes.)

So here's my question: When does Facebook hit the next milestone? And the one after that?