Boyfriend Troubles

Recently I got on my boyfriends facebook, I wasn't planning to snoop, but I noticed he kept getting messages while I was scrolling through the news feed. I ended up looking at them and he had been messaging two girls. The conversation(s) seemed innocent from what I saw, but he kept deleting the messages after a few responses (i wasn't able to see full conversations just bits and pieces). He's never talked about these girls before and by what I read they haven't been long time friends. Should I be worried? I know I should ask him about this but I am not sure how. Him deleting messages just makes it suspicious especially since he doesn't do that with other messages just from those two girls.