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In January a pair of money managers, Fidelity and T. Rowe Price, bought 9.1 percent of Slide for $50 million. Fortune asks, "Are these widgets worth half a billion?" The mag doesn't come up with anything more than "maybe," but I'm willing to go a little further. Slide worth $550 million? No, despite its huge traffic numbers. While it's true that advertisers are desperate to reach the 18-24 market, I hardly think SuperPoke is what they had in mind.

Slide won't be running an IPO any time soon. The only way founder Max Levchin and Fidelity and T. Rowe Price cash out is by being acquired, perhaps for a hefty chunk of Facebook stock. Mark Zuckerberg's company already has a huge amount of eyeballs, but picking up Slide would give them even more — and most importantly, massive reach across the other social networks that Slide's widgets run on.

Zuckerberg has already said he wants to expand Facebook across the Web, looking ahead to the inevitable day when growth on his site stagnates, the way it already has on rival MySpace. Beacon, his first attempt to extend Facebook, flopped last year amid charges that the privacy-invading feature ruined some users' Christmases.

An ad network for widgets could be Facebook's answer to Google AdSense. Google makes a ton of money from ads placed on, but reaches thousands more sites by offering them a cut of ads it sells and places. It also helps track users as they move around the Web.

Combining Facebook's existing ad ventures with Slide's huge audience of drunken-party-pic posters would give Zuck a hedge against fickle users abandoning his site for the next new thing. As Google showed us, the advertising business is where the money is made. It would be smart for Zuckerberg to solidify his hold on the market while he still can. Of course, he'd have to buy Slide for inflated stock, not cash — but Web entrepreneurs like Levchin love to deflect reporters' questions about their wealth by saying it's all on paper.