I used a $100 Google Adwords coupon to advertise Dropbox on Google with my referral link. I did it to proof it could be done, not that I needed the extra space. Many others have done the same in the past http://www.lifehacker.com.au/2011/11/max-out-your-dropbox-re...

I won't speak for the parent, but, some people have been know to use Virtual Machine that pretend to be a "Friend" and take advantage of Dropbox's customer acquisition technique.

You get space when linking your Dropbox with Twitter, and things like referring friends. If you participate in the yearly challenges, you get space for every step you complete successfully.

I do not know if it is still the case, but if you used the "Camera Upload" feature in the past you could get extra space at 500 MB for each 500 MB uploaded up to a certain capped amount. When the feature was in beta it was capped at 5GB extra, but before it went stable they lowered the cap to 3GB.