Facebook’s first president had some choice words Wednesday for the social media giant he helped to found. Sean Parker, 37, criticized the potential effects of social networks and said he was concerned about their effect on human psychology.

Parker made his mark as an entrepreneur early: at 19, he founded the music-sharing service Napster. At 24, he became the founding president of Facebook. Parker is now worth some $2.6 billion, making him the 93rd richest person in tech and the ninth richest entrepreneur in America, according to Forbes.

Now the founder and chair of the Parker Institute for Cancer Immunotherapy, Parker recently put $250 million of his money behind cancer research, making him the largest donor ever to immunotherapy treatment research. While speaking Wednesday about cancer research during an Axios event at the National Constitution Center in Philadelphia, Parker said he has become “something of a conscientious objector” regarding social media.

“It’s a social validation feedback loop,” Parker said. “It’s exactly the kind of thing that a hacker like myself would come up with because you’re exploiting vulnerability in human psychology.”

Parker also said he was concerned about the potential impacts on kids.

“It literally changes your relationship with society, with each other,” he said. “It probably interferes with productivity in weird ways. God only knows what it’s doing to our children’s brains.”