1. If the DA is investigating a crime, such as pandering, pimping or solicitation, the right to subpoena is available through law.

2. Nothing.

If the fact that the ads are "private" meant that they could not be subpoenaed, then subpoenas would become pretty meaningless.  After all, if the information is public then the D.A. can get it on his own.

A prosecutor can subpoena pretty much anyone or anything he wants, so long as he can demonstrate that the subpoena is reasonably likely to lead him to admissible evidence in an ongoing case.  It is not necessary that records turned over by the subpoenaed entity be admissible in court; the prosecutor need only show that the records reasonably might help find other information which could eventually lead to admissible evidence.

When an organization receives a subpoena for private records of an individual, it is supposed to notify that individual so that he or she can challenge the subpoena in court.  (I should add that subpoenas generally allow at least ten days before the records must be produced, and this time provides an opportunity for the people whose privacy is at stake to bring a challenge.)

If a web site receives a subpoena it must comply unless the court or the D.A. say otherwise.  If the site properly notifies the lady or ladies involved and they don't challenge the subpoenas, then the site is off the hook.  The same is true if the ladies do bring a challenge and fail.  

The site could open itself up to a civil lawsuit if it fails to properly notify the ladies involved or if it turns over records when it has been notified by the D.A. or the court that it should not.  However, disreagarding the subpoena is a very bad idea, as the site and its key personnel could be held in contempt and, under some circumstances, could even be prosecuted.