Ever wonder why you can’t poke around anywhere on Facebook without running into a ton of baby pictures? It’s because there are just so many people on the social network in the first place, there’s bound to be a high proportion of cute photos. That’s my theory, but really there are a lot of people on Facebook now — the site has reached one billion users.

Facebook has been struggling to show its investors just how awesome it can do with online ad revenue, so having a number like one billion users to wave as a potential pool of people to market to is probably making the company pretty happy right about now.

Mark Zuckerberg has come a long way since sitting in that sophomore dorm room putzing around on a computer, and he was the one to make the announcement today in a posting on his site.

“If you’re reading this: thank you for giving me and my little team the honor of serving you. Helping a billion people connect is amazing, humbling and by far the thing I am most proud of in my life.”

According to a Q&A snagged by Bloomberg, the Facebook family all crowded around the metaphorical fire and watched the user countdown (countup?) clock together. Okay, so it wasn’t that romantic and we might never know who the actual billionth user is.

“So it’s funny, we were all sitting around watching us get to a billion users but it was actually just a sample of the users. It’s like you’re not going to try to pull a billion rows from a database, so you’ll pull a sample and project out. I don’t even know if we knew who the billionth person was.”

After that he says, they all went back to work.

The next time an advertiser is debating whether or not to go with some Facebook ads, The Zuck can just say, “Ya know… there are a billion monthly users on this baby.”

Now, let’s see who can get to a billion friends first.

Facebook’s ‘Next Billion’: A Q&A With Mark Zuckerberg [Bloomberg]

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