Chad (disclaimer: I work for Balanced https://www.balancedpayments.com/ and we handle the payment processing for gittip. I mainly focus on fraud). It's wonderful you are doing this. It enable forms of donation other than via github. Not sure, what information you collect via twitter login, but I would watch out for:

1. Twitter account creation date 2. Number of followers/following 3. Location information (if available) 4. # of tweets

Esp. (1) and (2) would be very useful. When I last analyzed the github accounts, almost all fraudsters had relatively recent github accounts and (unsurprisingly) zero/almost zero followers for their repos. As with most things related to fraud, these do not guarantee fraud protection, but serve as very strong signals.