Some interesting developments in Social Networks being gobbled up. In this case, a couple of Amazon.com acquisitions means it now owns two competing networks:

Amazon.com Monday agreed to acquire Shelfari, a social network for book lovers, for an undisclosed sum. Amazon’s acquisition of Shelfari means the site will likely make it a much stronger competitor to other social networks that focus on bibliophiles, according to some observers. In addition, Amazon earlier this month acquired online rare book seller AbeBooks, and gained its 40% stake in one of Shelfari’s main competitors, LibraryThing. Thus, observers note, Amazon will have a stake in two competing social networks for readers.

Amazon’s buy-vs-build strategy to build up its strength in social networking is likely the start of a trend. Social networking sites who have been able to find the right formula and build their base will be attractive properties for allied e-commerce sites.