Is he ignoring me?

Okay so my ex and I said that we would continue to be friends even though I'm away at school and he's back home. So last month, he and I hung out and one thing led to another and we had sex...after that happened, he kind of just stopped talking to me and now I'll text him like hi or something and I won't get any answer at all. I'm thinking his phone went out of service, but I'll see him post on Facebook...do you think I'm being ignored? Should I reach out to him through Facebook? What should I do?