Tim O’Reilly has some interesting observations concerning Amazon.com‘s acquisition of Shelfari earlier this week—starting with the fact that the current playing field for book-themed social networking appears to be dominated by Goodreads.com, and he’s got a chart to prove it:

“Of course, that could change quickly if Amazon throws their muscle behind Shelfari and integrates it into their overall service,” O’Reilly notes, citing a trend where “companies with dominant share tend to get more dominant over time,” even when moving into new but related markets. “But here’s the counter,” he adds: “open and interoperable applications, including open social networks.” What if all the other social networks dedicated to books banded together and let their users communicate with each other over the fence, as it were?

O’Reilly doesn’t mention this, because his main emphasis is on the survival of the networks, but… the consequences of such a move for authors and publishers, who currently have to choose one bookish network upon which to focus their attention or else scramble to maintain some sort of presence on several of them, could be significant.