Facebook's privacy rollbacks may have pissed off users, but the bottom line has been just fine. That may yet change. CEO Mark Zuckerberg has acknowledged that the social network's rapid growth has finally slowed.

In an interview with Inside Facebook, Zuckerberg was asked about his rival Twitter. Zuckerberg said he became a bit too obsessed with the microblogging service until he noticed its insane growth rate had slowed. The same thing, he said, happened to his own social network:

We saw our exponential growth rate continue for a very long period of time, and it still does at a super-linear rate, though not quite 3% a week any more.

That's not a damning statement about Facebook; in the same interview, after all, Zuck basically confirms Facebook is on track to do in the neighborhood of $1 billion in revenue next year. But the slowdown admission follows months of controversy over Facebook's various privacy breaches and rollbacks, an outcry that had Facebook watchers speculating the social network's wildfire days might be behind it.

Example: In May, after a rash of bad publicity for Facebook, Danny Sullivan of Search Engine Land dug through Facebook's old user-count announcements to make an intriguing case that growth had dropped off sharply. The social network bragged in April about adding 10 million+ users in three weeks. Generously extrapolate this into monthly growth of 15 million users, Sullivan pointed out, and it's still weak:

  • August 26, 2008: 100 million
  • January 7, 2009: 150 million
    (50 million gain over about 4 months = 12.5 million per month)
  • April 8, 2009: 200 million
    (50 million gain over about 3 months = 16.7 million per month)
  • July 15, 2009: 250 million
    (50 million gain over about 3 months = 16.7 million per month)
  • September 15, 2009: 300 million
    (50 million gain over about 2 months = 25 million per month)
  • December 1, 2009: 350 million
    (50 million gain over about 2.5 months = 20 million per month)
  • February 4, 2010: 400 million
    (50 million gain over about 2 months = 25 million per month)
  • [April 2010: 10 million gain over 3 weeks = 13-15 million per month]

Facebook is on track to hit 500 million users any day now. No one's saying the company will collapse or relinquish its spot as the dominant social network. But Facebook is leveraged using venture capital, and its backers, some less friendly than others, will surely be concerned about any significant deceleration. The only question is whether the compny has alienated a "significant" chunk of potential users yet, or whether that's still a few more messy privacy controversies away.