Yeah, the idea of companies becoming capable of widespread surveillance because of the size of their data set is also bothersome to me. But because it is so hard to define what that fuzzy line is (between an acceptable amount of data and too much data), it's too arbitrary for me to draw a line in the sand at all.

Which might seem wild, but it's actually pretty consistent with the amount of trust we put in companies right now. Facebook could arguably do some nebulous bad thing with their facial recognition data. AT&T can definitely know who you are calling on any call that goes through their POTS system. But we don't have a ban on diagnostic tools for large telephone providers out of fear that they'd abuse their position - I see no reason to ban a technology of large social network providers for the same fear. [0]

It comes down to trust - as soon as one of these companies did abuse their position to do something widely evil, the market consequences would be dire (to say nothing of the reactionary legislation).

[0] Possibly a poor example, considering anti-trust w/r/t Bell growing too large - but that kind of regulatory intervention I would hope could be applied equally to either class of company should they be determined a monopoly.