If everyone involved consents adequately, this would be legal (assuming, of course, that the data isn't being collected as part of a scheme to defraud the users).
The United States has few data protection laws (and those are mostly at the state level). None of them prohibit consenting to share data for a fee.
This would generally be taxable ordinary income, although there is room to split fine hairs over whether it is as independent contractors or is a royalty on the intellectual property of the users (which would matter for purposes of the self-employment tax imposed in lieu of FICA taxation on self-employed people).
A royalty treatment (which would not be subject to self-employment taxation) would probably be more correct than an independent contractor treatment, since this is a payment for intellectual property that belongs to the user and not a payment for work done by the user or goods sold by the user.
It might be possible to structure this as a rebate on the membership fees for subscriptions to the social media site if there was one.