Zuckerberg himself donated to Diaspora, so I don't think he'd care.
You're not going to see a startup get accepted that is essentially a competitor to Facebook -- it's too late for that. Even if it were, this partnership would give them the chance to know about the startup and acquire it long before it becomes a threat.
How about if a startup works with YC and FB, will they be prohibited from working with Twitter? What if FB is developing a "beta feature" that the startup wants. Can they ask Twitter for the same feature?