Dropbox really is a great example of invites done well. Google's "limited invite" comes with a sense of smugness as you get something others don't. Dropbox's

Dropbox really is a great example of invites done well. Google's "limited invite" comes with a sense of smugness as you get something others don't. Dropbox's referral program is on the spammy side of things though, I've seen people complained about it on Twitter. I think the key in referral program is to reward your loyal customers who went out to sell your product for you. Make them part of your business, give them the power to give out freebies, but put a limit on it to create prestige... (a mix of Dropbox & Google) The other thing is viral pressure... and nobody does it better (or worst depending the way you spin it) than Zynga. There's a public Google Doc named "What I learned from Farmville so you don't have to play it", really good read on its viral pressure mechanism! Not sure if it applies to your business but will probably get you thinking nevertheless.