Another data point: I obtained nearly 30 referrals this way for a total of ~$40.00 USD. Another 12 people signed up but didn't install the desktop client. I said "Free Dropbox Account. Get 2.5GB of storage space. Signup takes 60 seconds." I limited the ad to the United States and only keywords with Dropbox/drop box in them. I e-mailed everyone who registered but didn't complete installation with a "Don't forget, you have to install Dropbox to get the rest of your free space!" welcome e-mail. Sadly, I didn't include a trackable link but about 25% of the stalled out referrals went on to complete, saving me several dollars.
I think Dropbox realizes I captured some people they might have otherwise lost and that unless I max my space, they have given me nothing.
Dropbox's business model is successful because they give you 2GB space but you can guarantee that most people will only use a fraction of that.
I took a similar path and decided to email those who signed up but didn't "complete" their registration with a download - it doesn't appear to have been successful, but time will tell.
I just checked my referral log. It's been a few months now and it looks like either one or none of them have converted since I hit the limit. I may e-mail mine a second time to see if that converts a few more. When I e-mailed my referrals the first time, it was less than 24 hours after they signed up and I think that's why I saw as many conversions as I did.
I think Dropbox realizes I captured some people they might have otherwise lost and that unless I max my space, they have given me nothing.