Change is possible, but it will not happen if we all do nothing and wish someone else would have done something.
This feature was down as "coming soon!" for a long time, but as far as I can see having had a quick look at the site for the first time in months, and that feature doesn't seem to be mentioned at all now.
This is a key problem for projects like this: many people who are interested in a particular aspect won't touch the project until that aspect is mature, or at least present, because if it isn't there already it might never turn up as project priorities changes (OK; so existing features can vanish over time for similar reasons, but this is much more rare).
EDIT: Just found mention of the above feature buried in the FAQ (it used to be on the front page or there-abouts), and it is now described as "in the future" not "coming soon" so I shall not hold my breath. Going by that document the "export all your data" half of the feature is present though, which would at least satiate my paranoid need to keep my own backups of everything.
I hope that diaspora will develop faster, now that it is a completely community run project and has some momentum again.
About your specific problem: You can host your own diaspora pod if you are really paranoid, for free, on Heroku for example. But I agree that this is one of the basic features that should be implemented as soon as possible.
I would imagine some active users post on both, and most users would just use Facebook. So the users of both will be annoyed to get double updates for some people. The users of one of them will wonder why the other friend knows more about their mutual friend. I can't see getting into Facebook space as a good thing. For example Twitter seems to have a different space, Instagram also seems different.