How can it be Sustainable? Kiva's Social Lending Campaign

I am confused how Kiva's Social Media lending campaign can sustain itself if it goes viral. Quick rundown on the campaign is that if you get a referral link, you post it on FB/Twitter etc. For every person that signs up you get $25 to lend and the referrer gets $25 more to lend. If my referral link gets a thousand sign ups, then Kiva is responsible for $50,000 worth of loans. Is this user acquisition strategy sustainable? Its not like they are giving out free software trials. I suppose the hope is that people signup and then throw their own money into the pot and continuously lend over time thus Kiva earns interest over time to make up for this. Yet it seems like the amount of money could balloon in this campaign really fast. Here is my Kiva referral link via Facebook: http://bit.ly/PEtET3 Non-Referral: https://www.kiva.org/portfolio/invites In case you don't know Kiva - its a Micro-financing platform - signup and choose a third-world person/cause to lend to. Maybe the loan is repayed, maybe not. If it is you can lend again.