He did not invest in porn, and that would not make him a criminal anyway. He did own a "Minitel Rose" company which offered erotic chatting services over Minitel, the french precursor of Internet. After getting rich with it, he sold it and started a new company named Free, a company offering a free internet access services.

This has nothing to do with criminal activity. Running an erotic chatting service was a perfectly legal activity in France at the time. It was latter refrained and maybe the reason Xavier Niel changed his business activity.