How should I respond to this facebook message?

[Dance]

Hello. In high school I had friends , but I was not popular due to my "loser" status in middle school. I I even had bouts of bullying that contributed to PTSD that sadly I am still dealing with. Flash forward to the present and I am 21 years old ( so its been a while since high school lol ) , and I have been very successful and accomplished a lot and I am still successful. Yesterday at about 3 am I got a text message on Facebook from a boy who was part of a group of people that behaved very disgustingly towards me. He said" heyy". How should I respond to this FB mesage and wy do you think he sent me this message?
on a side note : he is already my FB friend, has been since high school. I also run a public blog so people see my stuff on FB and they can see my travels etc.