An easy way to deal with this would be to say, preferably in the promotion video, something like "if we receive the desired $10000 amount then no further donations will be accepted", or to nominate a charity which any surplus donations would go to. If they receive a large amount of money and then don't deliver much at the end of the summer then there could be a lot of negative publicity.

But I wish the diaspora developers well, and hope that they deliver something which is at least a foundation for a privacy respecting social network in future. Clearly there is a need for some software filling this role, and enough people are fed up with the status quo to put money towards a solution.