> acquiring Fitbit will just mean they'll have that same data for a whole bunch more people
And this is why you have to carefully read privacy policies. Pebble had a clause in theirs saying they could sell any personal data about you to anyone, either as part of a company acquisition or for any other reason. Pebble got bought out by Fitbit. If that data is part of Fitbit's sale then Google will now have all data Pebble ever collected on anyone (which was, at least potentially, a lot).
What they currently do with the fitness data they collect is probably a pretty good guidepost for what they'll continue to do with fitness data they collect.