I've looked at Shelfari several times over the course of its existence, and it always seemed like a cute project that would never make it big. Sometimes, however, it's not only about the size of your community; the big players in your corner of the market can make a big difference.
Thus, Shelfari has been acquired by Amazon; probably not because it has a huge community, but because Amazon needed a book-oriented social network and acquiring Shelfari was the easiest, fastest, or least cash intensive way to do it.
I'm not usually thrilled about acquisitions like these, but in this case I think that Amazon will actually help Shelfari to grow. It'll be much easier to build a community for book-lovers if you've got Amazon's huge user numbers and book inventory to help you.