Actually I add my books to all of them 
BookBrains because I love that all the short fiction and articles have their own entries (and I should mention it, it has the ‘ahem’ nicest community 
)
LibraryThing for managing (sorting, tagging) my libary and
OpenLibrary because I can add my specific edition + a scan which I link to the respective BookBrainz edition (unfortunately BB does not have this feature yet).
So they all have their benefits…