And that's fine. But because they'll commit to integrating into the open standard that Diaspora defines, at any given time, you can take your friends and leave.
If GMail (apps) starts messing with the privacy of my e-mail, or wants me to pay more than I want to, or changes the layout to something I don't find pleasing, instead of joining a group complaining, I can leave them in a heartbeat. The people I exchange emails with won't notice a bit. The promise of Diaspora is giving me that option for social networks.
There will be a GMail, there will be an Exchange server and there will be a buggy open-source self-hosted PHP-app (and many many more) in the Diaspora universe.