I don't think so. Their stated objective is to change the world's social networking platform. Not start a lifestyle business or pet project. If they had gone into it with a hobbyist's approach 170,000 users is success.
What if it ended up that Diaspora managed to gain enough users and convert enough of Facebook's user base to gain about 17.5% market share? (Measured in time online, on-site. Probably hard to measure, but this is a hypothetical.) Would they be a success? What percentage would represent the threshold?
Based on that, I would say success is 1) different things to different people, and 2) the fulfillment of each person's goals.