A subpoena requesting documents relevant to an investigation is presumed reasonable under the 4th amendment. Generally, the government can compel you to turn over information relevant to an investigation of someone else. The subpoena power is very broad and also very old (predating the 4th amendment).

If the government hacked into Facebook's servers and took the data, that would be a violation of Facebook's 4th amendment rights, but it still wouldn't be a violation of yours.