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First came Facebook's marketplace feature, a challenger to Craigslist's for-sale listings. Now the social network is taking on filthy-rich Craig Newmark in another key attractor: the couch-surfing market. Travel search-engine SideStep has signed up on Mark Zuckerberg's team by building a Facebook app, CouchSwap, to target the broke college-student traveler. You can search for a couch by location or send out special requests — a much more efficient way to go about locating crash space than trawling Craigslist listings. And after a successful crash, users can rate their stay. If you want to participate, though, you'll have to be willing to share your living room, too — a requirement Craigslist doesn't impose. The dark horse in this race: The established CouchSurfing.com community. (Photo by Erik Abderhalden)