Shelfari, a social network for bibliophiles, has been acquired by Amazon for an undisclosed amount. Amazon has been a longtime supporter of the Seattle based startup, having invested $1 million in the site in February 2007.

The move comes less than a month after Amazon’s acquisition of AbeBooks, a vendor of rare and used books from independent publishers. As part of that acquisition Amazon also got a stake in Shelfari’s competitor LibraryThing, which AbeBooks had previously purchased a 40% stake in.

This resulted in an awkward scenario – while Shelfari and LibraryThing are similar and could conceivably be merged by Amazon pending a dual aquision, there is bad blood between them. LibraryThing’s founder has openly criticized Shelfari for spamming users and astroturfing blogs, and generally behaving as a “bad actor”.

In light of the tension between the companies (and their locations on opposite sides of the country), we speculated that Amazon was going to have to choose to acquire one of the competitors and divest its shares in the other. Amazon has clearly sided with Shelfari, though a later acquisition of LibraryThing is still feasible.

You can see Shelfari’s blog post on the acquisition here.