Seeing that social networking isn't really paying the bills, Facebook has gotten into file sharing. Well, not really, but it is making file sharing a core feature of groups. The latest incarnation of Groups enables people to create a community around a common interest. They can share stuff inside the group and do most of the stuff they can on Facebook with only the people in the group seeing it.
Now, Facebook is adding file sharing to the list of things you can do in a group. This isn't the second Dropbox and it isn't the second Pirate Bay, if anything, file sharing in Facebook has more to do with Gmail attachments.
Users can upload files, as large as 25 MB - which isn't very large, and make them available to anyone inside a group. Executable files are not allowed (viruses and all that).
Neither are music files because, well, that's evil pirate stuff and Facebook is a respectable establishment (which doesn't want the music labels piling down on it).
Everything else is fair game, ebooks, documents, very short videos and so on. And if, somehow, evil malware or illegally shared copyrighted files do make it through, you can report them as you would any other nefarious content on the site.
Obviously, the feature isn't going to be able to compare to Dropbox any time soon, but it doesn't have too. Facebook Photos was slow, only displayed small photos, the uploader was buggy, and it lacked much of the basic tools that photo sharing sites of the time offered and it still trampled them all.
Facebook's sharing feature only has to be good enough and it will find an audience of millions, even if it's a huge step backwards towards email attachments, mismatched file versions and so on.