So Amazon has bought a chunk of group-buy platform LivingSocial, eBay is trialling a group-buy Groupon clone (screenshot below), Google is launching a Groupon clone, and Microsoft is running a JV with another Groupon clone and using Groupon itself to shift Windows phones (screenshots below).  Now, who haven’t we mentioned?

Facebook.  Well Facebook has just revealed it is trialling its own group-buy social commerce product called “Buy With Friends”, which will give users discounts on products that their friends have already purchased.  It’s an interesting, fresh and creative build on the much emulated Groupon format. Discounts and terms will be set by participating vendors.

In early Buy-With-Friends tests, restricted to Facebook Credit purchases of virtual goods, purchase notifications are shared in newsfeeds to friends. Seeing the purchase, friends can make the same purchase but at a discount, and directly from their newsfeed.  Deb Liu, Facebook’s head of commerce product marketing, claimed at this week’s Inside Social Apps Conference in San Francisco that more than half of users chose to share their purchases with friends.

Facebook’s Buy-With-Friends social commerce app is, of course, in addition to numerous third-party group-buy apps already available in Facebook, and in addition to Group-Buy deals that retailers can already set up in Facebook Deals.  But we think Buy-With-Friends has the potential to be big, very big – partly because it harnesses the viral potential of Facebook, but also  it because it allows people to buy directly off-the-wall, and uses easy-to-spend Facebook Credits (which will soon be compulsory for Facebook hosted games).  When you factor in the announcement yesterday that Facebook is to get more secure with HTTPS and a smart, social photo identity (tag-a-friend) test, the prospect for Buy-With-Friends is bright.

Initially, it would appear, Facebook’s Buy-With-Friends social commerce app will be limited to offers to the purchase of discounted virtual, not physical, products. But we expect that to change.

Group-Buy: The Hot Social Commerce Trend for 2011

Microsoft Selling On Groupon

Google Offers

Microsoft’s Cudo

eBay Group-Buy (India)

Are there any big players left not joining the group-buy social commerce trend?