Trouble in paradise

My husband and I have been married for almost 4 years and have been together almost 8. Recently I have been noticing red flags and it has made me curious so I didn't what I think no woman or man should ever do to their spouse and spied on his facebook. But what I saw was not what I wanted to see. He has been messaging women in the local area and it has not be hey buddy how are you kind of conversations. When I ask him about it he shrugged it off and pretends I didn't say anything. And then I just wondered wonder if he is still up to it... and there again he is just talking away to them like he isn't even married or connected to anyone. I just don't know what to do. I don't want to lose the love of my life and best friend... but I just can't do it anymore. I need someone to give me their unbiased advice.