Groupon and LivingSocial and their billions in projected valuations are under attack.

Tech giants have been gunning for the lucrative flash-sale niche for months. AOL (NYSE: AOL) launched Wow.com late last year. Google (Nasdaq: GOOG) is offering advertisers the opportunity to sell discounted vouchers. Amazon.com (Nasdaq: AMZN) would have been all over this if it hadn't acquired a meaty stake in LivingSocial three months ago.

However, the real potential Groupon gouger -- the one that can turn LivingSocial into DyingSocial -- is Facebook. The leading social-networking site has a dynamic grip on most of its roughly 600 million registered users. It's viral. It's sticky. If Facebook should ever throw its hat into the group-buying ring, it can really be a game changer.

What's that? Facebook's already here? It's about time!

Facebook is beefing up its Deals website. It is now offering localized deals in Atlanta, Austin, Dallas, San Diego, and San Francisco. "More cities coming soon" is the tease for greater geographic expansion. Sign up for local deal notifications and the announcement can go out in your news feed, triggering the viral sweetness that makes Facebook the tastemaker juggernaut it has grown to become.

This looks like trouble for Groupon, LivingSocial, and BuyWithMe -- as well as OpenTable (Nasdaq: OPEN), Travelzoo (Nasdaq: TZOO), The Knot (Nasdaq: KNOT), Yelp, and any other niche-specific site that hopped on the Groupon bandwagon.

The stakes are high. Groupon reportedly rebuffed a $6 billion buyout offer from Google. LivingSocial is looking to raise capital in a deal that would value the silver medalist at a cool $2 billion. 

Facebook's ability to blend Deals with Places -- offering nearby deals at destination check-ins -- could be enough to torpedo both Foursquare and Groupon. Unlike the giant search engines, free-mail sites, and Web portals with similar intentions, Facebook has already cracked the social code. It's already hanging out with your circle of local friends. Why do you think Groupon, LivingSocial, and any flash-sale site with a pulse want you to share your voucher purchases on Facebook? 

Facebook is already where everybody else wants to be. Snipping out the middleman is the social giant's most lucrative use of scissors yet.

Will Facebook dominate this niche? Share your thoughts in the comment box below.

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Longtime Fool contributor Rick Munarriz wonders if social coupons will be a passing craze or if thriftiness is forever. He does not own shares in any of the stocks in this story. Rick is also part of the Rule Breakers newsletter research team, seeking out tomorrow's ultimate growth stocks a day early. The Fool has a disclosure policy.