You might wish to review their Privacy policy, which inspires more confidence than Goodreads:

https://www.librarything.com/privacy

The site's also been around since 2005, so, that speaks well to both its longevity and its integrity. Of course, that's not to say they couldn't be acquired tomorrow and paywalled, but I think it unlikely.

I don't think I am particularly worried about a proprietary solution. If I am putting my preferences in the public, pretty sure I am giving up on some dimension of privacy. With books at least, it doesn't seem so concerning as long as the recommendations don't suck too much (I don't particularly rely on recommendations that much though). But of course, you never know!