I do believe in presumption of innocence, however, if this guy truly was innocent then we have a much bigger problem than mob justice in jail. For him to be innocent the public is falsely reporting people to the authorities for exposing themselves to children and the police are carrying 'throw-down' kiddie porn so they can frame innocent citizens. It seems much more likely to me (way beyond a reasonable doubt) that this guy was a sexual predator than that the police and the public were involved in a conspiracy against him. That would be a scary society.
Regardless, the system is in place for proper prosecution of criminals. Mob justice is no answer. Allow it once.. allow it again... who's to say if the mob was correct?
The police wouldn't have to be involved in such a conspiracy, although in this case a setup by a private actor would be very difficult (though not impossible). And yes, people do falsely report incidents to police all the time. The problem is that the man never got a chance to defend himself. He never got a chance to point out things that could prove that it was a setup.
Was the man guilty? Possibly and perhaps even probably. But without examining the evidence and hearing the defense there is no way I could say that I think he's guilty beyond a reasonable doubt.