My school library is the fifteenth largest library on LibraryThing.com. One reason why is that there are fairly severe file size limitations for uploads. The first time I tried to upload the file was too large. I had exported too much data on each book. The second time I only exported the ISBN numbers and it worked just fine.
I suspect that is why my small school library, about 10,000 books, is that high up the foodchain. Larger libraries have difficulty uploading their data.