The question is, if you hold a garage sale and someone buys your stuff and the governments then buys that stuff from whomever you sold it to, is it a Fourth Amendment violation?
Because that's what's really happening here. Once you sell your data to Facebook, Twitter, etc, it's no longer "in your house." If you granted third parties license to collect and sell your data under terms of service, then that data no longer belongs to you, and you agreed to that..
To me the bigger problem isn't the government buying data on the open market, it's that data is a market to begin with.