I'm guessing that: a "document" on Twitter is a single tweet; a "document" on Facebook is a wall-post or equivalent; a "document" on GMail is an e-mail; a "document" on Google Calendar is an appointment.

Therefore, the comparison with Google’s web-wide index in 2001 is a little misleading (in terms of the amount of data), given that the average size of a web-page is greater than all of these.

Of course average size of a file on Dropbox is likely to be larger than a webpage. I wonder what percentage of those 1.5 billion documents are files on Dropbox.