Has PA considered adding a referral program where paid users could receive small discounts on their monthly fees if they refer users that sign up with a paid account? Before upgrading to a 100GB paid Dropbox account, I referred 32 friends to sign up for free accounts with Dropbox, eventually capping out my referrals and accumulating 16GB of space.
Even though I love PA and speak highly of it to friends and on the interwebs, if I had a stronger incentive to get people to actually sign up for paid accounts, I would push harder. I'm not sure what the number (or marginal features?) would have to be to make it worth it, but I know that number exists and is less than the cost of a $5 account.
Signed, Proud owner of 4 paid PA accounts.