Facebook sure has put on a lot of weight this summer. Over the past two months alone, the social-networking site has packed on another 50 million registered users.

Yes, Facebook is taking over the world.

It was mid-July when the company was celebrating a new milestone: A whopping 250 million users. Yesterday it announced that it passed the 300 million mark. Summer may be the seasonally sleepy chunk of the cyberspace calendar, but it's hard to keep a viral trend down.

The real nugget in Mark Zuckerberg's mile-marker announcement is that Facebook is now taking in more money than it's spending.

"Earlier this year, we said we expected to be cash flow positive sometime in 2010, and I'm pleased to share that we achieved this milestone last quarter," he writes. "This is important to us because it sets Facebook up to be a strong independent service for the long term."

Did you catch that? Zuckerberg's blog entry is intended for "fans" of the site. How many do you think grasp the concept of cash flow? Not many, one would think. This feels more like it's telegraphing an IPO to prospective underwriters. If not, then Facebook is ringing the dinner bell for Yahoo! (NASDAQ:YHOO), Microsoft (NASDAQ:MSFT), or any potential buyer.

One can argue that Zuckerberg is too proud to cash out. He also told Reuters that any potential IPO is still a few years away. Then again, he said that back in May -- Facebook was just a 200-million-member baby at the time. We still didn't know if the market rally was sustainable.

Facebook would be doing Wall Street a favor by going public now, giving it a sexy name to chew on.

The social-networking sites that are part of public companies -- News Corp.'s (NYSE:NWS) MySpace, Google's (NASDAQ:GOOG) Orkut, and Time Warner's (NYSE:TWX) Bebo -- are yawn-inducing laggards compared to Facebook.

One can always argue that a cash-flow positive Facebook doesn't need to go public, but the company clearly needs to find a way to attach a real value to the stock options it's offering key hires for retention purposes. The mile markers will eventually slow down, so it may as well strike while the accelerator pedal is hot.

It planted the IPO seed yesterday. Now let's see if it remembers to water it.

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Longtime Fool contributor Rick Munarriz remembers when social networks were an offline endeavor. He does not own shares in any of the companies in this story. He is also a member of the Rule Breakers analytical team, seeking out the next great growth stock early in its defiance. The Fool has a disclosure policy.