Casual meeting evolves into creepy obsession

DEAR ABBY: I recently met this guy at a private event. We got to talking, and he gave me his social media information. We have been messaging each other ever since. He has been acting increasingly obsessed, and it’s making me uncomfortable. He acts like we’ve known each other for years. I don’t know how to tell him I don’t want to talk to him anymore. What are your thoughts? — RESERVED GUY IN MASSACHUSETTS