Depends a bit on the clientele you're serving I guess. At first, the referral part (encourage others to upgrade your account) worked very well, I saw people either onboarding their other accounts, friends' accounts, or just heading straight to Twitter to let everyone know they're recommending the service (so their account would upgrade for free). So that brought users.

The "buy in bulk" was silent for a while, until a few freelance social-media managers discovered that they could buy account slots for their clients in bulk here, and then charge their clients for that plus a fee on top (which I didn't regulate). So that surely brought the biggest chunks of purchases albeit irregularly so.