Hello, inevitability. Deals for Facebook launches today in Atlanta, Austin, Dallas, San Diego, and San Francisco, offering users the opportunity to cash in on city-specific discounts through prepaid vouchers. The Deals tab has been active in those cities since last month, but the actual discounts start rolling in today.

Somewhere out in Chicago, Groupon is kicking itself for not pulling off its IPO sooner.

Sure, Groupon had its share of flashy wannabes temporarily strutting their stuff. Beyond Amazon.com (Nasdaq: AMZN)-backed LivingSocial, no other site has really challenged Groupon. AOL (NYSE: AOL) and now Google (Nasdaq: GOOG) are rolling out local deals, but what those sites draw in traffic, they lack in stickiness.

Groupon broke away from the pack because it understood the viral nature of Facebook promotion. Where would the site be today without its ability to wedge its deals into Facebook user news feeds by incentivizing the original buyer?

Facebook is automatically Groupon's biggest threat, because it controls the same infrastructure that helped Groupon succeed. Facebook can better encourage users to enlist their sprawling networks of friends to go on outings together, and use deals to champion its Facebook Credits virtual currency.

Intriguingly, Facebook isn't diving into this high-margin niche alone. Instead of setting up specialized sales forces in its launch cities, the site has simply teamed up with OpenTable (Nasdaq: OPEN), ReachLocal (Nasdaq: RLOC), and other existing, city-specific discount providers.

Is Facebook outsourcing these deals as a first step toward taking such services in-house? That's an odd strategy, especially if it winds up promoting the niche-specific partner sites it employs.

OpenTable has been on a tear since it began selling marked-down vouchers for dining experiences last year. Travel deals publisher Travelzoo (Nasdaq: TZOO) was trading in the teens when it introduced prepaid vouchers last summer, and its stock just cracked into the triple digits.

Facebook enjoys a huge opportunity here -- even larger than when it took on Foursquare with local check-ins last year. Where the latter service simply stoked geek vanity, the social network's deals offer real discounts on desirable experiences, provided by the gatekeeper that every other social coupon site has thus far relied upon.

Make way for Deals. It's going to be huge.

Are you looking forward to the inevitable Groupon, Facebook, or LivingSocial IPO? Share your thoughts in the comment box below.

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