So they know who was compromised and who wasnt? :)Thank you Seung, I hadn't heard about that.
For general consumption here is a link to an article on BBC news. Looks like this is the 2012 hacked data. If you are one of the affected users then you should find that your password has been reset next time you log on.
http://www.bbc.co.uk/news/technology-37232635
Tough to say, but I suspect that it may have been their entire user-base. 68 million accounts apparently. The 2012 dropbox blog post regarding the incident downplays it too much to get a good idea of the scope - https://blogs.dropbox.com/dropbox/2012/07/security-update-new-features/So they know who was compromised and who wasnt? :)