Amazon.com announced today that it is acquiring AbeBooks, a 13-year-old Victoria, B.C., online retailer of used, rare and out-of-print books.

Terms weren't disclosed, but AbeBooks will continue to operate its Web sites from Victoria. It employs more than 120 people and offers more than 110 million book titles, according to the Web site. It also has relationships with more than 13,000 booksellers.

The acquisition is interesting in part because AbeBooks' Web site -- launched in 1996 -- arrived on the scene around the same time as Amazon. (Amazon was founded in 1994 and launched a year later.)

The other interesting component is that AbeBooks is a part owner in LibraryThing, the social networking site for book lovers.

Amazon.com already is an investor in Shelfari, the Seattle startup that competes directly with LibraryThing.

What does this mean for Shelfari?

Perhaps it will force Amazon.com to purchase Shelfari or spin off the LibraryThing portion of AbeBooks. Whatever occurs, I am sure Shelfari's Josh Hug will be watching very closely.

Built with no venture capital financing and profitable from its earliest days, AbeBooks sold $190 million worth of books last year. (That's gross merchandise, with AbeBooks' revenue of about $30 million, according to TechVibes.)

UPDATE: LibraryThing's Tim Spalding writes in a blog post that Amazon.com will become a minority shareholder in his company after the AbeBooks' deal closes, noting that little will change with the site and Amazon will not be able to access user information.

Still, I wonder what Amazon will do with two investments in competing social networking sites for bibliophiles.

I have an e-mail into Shelfari's Josh Hug for his take.

UPDATE: It is unlikely that Shelfari and LibraryThing will be joining forces in a happy merger anytime soon, since there is bad blood between the two organizations.

Last fall, LibraryThing's Spalding described Shelfari as a "bad actor" for engaging in what he called a massive campaign of astroturfing." (That's the practice of planting positive comments about a service on blogs.)

Spalding said he came up with more than 50 examples, writing that "it's icky to ... go on and on about how much you 'love' Shelfari without mentioning you're paid by them."

In another post, Spalding accused Shelfari of being an unethical spammer.

"We respect our competitors with one exception: the site 'Shelfari.com,'" wrote Spalding, who went on to provide links to 51 news and blog entries calling out Shelfari's spamming practices.

Shelfari apologized and fixed the problems.

But I seriously doubt these two startups will overcome their differences even if Amazon holds a stake in both.

Venture capitalists typically don't invest in competing companies. Does that hold true to a strategic buyer like Amazon that picks up an asset as part of a bigger purchase? Could Amazon be forced to sell its stake in either LibraryThing or Shelfari?

Meanwhile, TechVibes estimates that Amazon.com paid between $90 million to $120 million for AbeBooks.

UPDATE: I heard back from Shelfari's Josh Hug, though he didn't shed much light on the potential conflict for Amazon directing me to the online retailer. Here's what Hug said via e-mail:

"I can't see how this will effect us. Amazon has been a very supportive investor and we look forward to continuing to work closely with them."