I was so excited when I discovered LibraryThing. I didn't even investigate the details, I just immediately opened a free account. Within a week I had paid for a lifetime account.
Never before, even in library school, have a met such a group of people who are well-read, conversational, pleasant, well-spoken, intellectual, and with such far-reaching interests. The other LT members have jumped right in to so many conversations with interesting suggestions, useful ideas, fascinating tangents, and even straightforward answers.
I won't really speak to the technical aspects of LT. I am one of the proponents of the advent of a collections feature so I could better separate out my own books from the children's books, and even more so that I could in good conscience include a collection of material I have read but do not own and of books to be read. I am a LT purist, and maintain a single account with a collection of books only including those in my physical possession. I do not like to try to use tags, which I see more as an indexing feature, as a collections management tool, but that is a personal bias and I know there are those who do just that. I actually am sort of fearing LT upgrades because I really shouldn't be spending any more time on the site than I already do.
I don't have accounts on other similar sites. I do (as is obvious here regularly) belong to BookMooch and also BookCrossing, but I don't use those at all in similar ways. BookMooch is strictly used in our household to get rid of books we really are not using (which are few) and acquire more books we will use (which are far too many). I have met nice people through BookMooch, but I don't look there for conversation, ideas, or networking. I don't even use their recommendations for the most part; I use LT recommendations and then link to my Mooch account to wishlist those. I view BC as an interesting social experiment but since it is such a small movement overall still today, I have yet to encounter a BC book "in the wild" and have only received one tagged book via BookMooch. I have released one book myself, and have another which I plan to release in the wild after I finally get around to reading it. BC is more about the books, though, and much less about the people.
The only other networking source I use so frequently is KnittyBoard. Of course, I am on so many email listservs that I get about 200+ emails a day and I interact with other people with overlapping interests in that manner, too.
I definitely recommend any bookworm check out LT! Have fun, discover more books, read, meet great people, and generally become obsessed.