Congress can compel any person or company to appear before them for any reason: this article and this more extensive review give overviews of Congressional subpoena power. Watkins v. United States, 354 U.S. 178 affirms the power of Congress to investigate matters that could bear on the actions of Congress, but also draws lines on what kinds of questions can be asked.
It is unquestionably the duty of all citizens to cooperate with the
Congress in its efforts to obtain the facts needed for intelligent
legislative action. It is their unremitting obligation to respond to
subpoenas, to respect the dignity of the Congress and its committees,
and to testify fully with respect to matters within the province of
proper investigation. This, of course, assumes that the constitutional
rights of witnesses will be respected by the Congress as they are in a
court of justice. The Bill of Rights is applicable to investigations
as to all forms of governmental action. Witnesses cannot be compelled
to give evidence against themselves. They cannot be subjected to
unreasonable search and seizure. Nor can the First Amendment freedoms
of speech, press, religion, or political belief and association be
abridged.
Congress has the power to enact laws (and it not charged with the duty to investigate federal crimes, which is in the domain of the executive branch). So if Congress wants to find out whether they should pass a law, they can subpoena people and ask them questions.
In the case of Watkins, an individual was interrogated regarding his personal life. He refused to answer, stating that "I do not believe that such questions are relevant to the work of this committee nor do I believe that this committee has the right to undertake the public exposure of persons because of their past activities". The Supreme Court concluded that while it is an offense to refuse to answer questions "pertinent to the question under inquiry", the Court could not figure out with reasonable certainty what the inquiry was about. BTW the holdings section of this opinion (in 1957) ranges a-y, which is a remarkably nuanced and non-committal collection of holdings, which can be summarized by saying that the court found the question to likely be "off-topic".
If Congress wants to contemplate passing a law to restrict Twitter, it can subpoena anyone it wants to. There is an unresolved question regarding subpoenaing the executive branch (specifically, the role of executive privilege, where POTUS can shift the burden of proof to Congress to establish their right to interrogate the executive branch).
