With Facebook integration, Dropbox makes it a lot easier to share something
Dropbox has added a small but quite useful and important feature in the long run, Facebook sharing. Now, instead of providing an email address of the people you want to share a file or folder with, you can simply pick from your Facebook friends.The move is important because the social component is what made Dropbox stand out of the crowd, the ability to share with friends and the free storage rewards for any of your friends that joined the site.
Using the new Facebook integration is simple, just create a new shared folder or pick an existing one and you'll see a link to invite people via Facebook rather than email. The new feature also works for existing share folders, you can invite more people via the social network.
Apart from convenience, integrating with Facebook should open up the service to a whole range of new people which may be very non-technical and would not find out about Dropbox or anything similar via any other way.