Boyfriend messages girl on fb alot?

Okay, so I have been with my boyfriend for 1.5 years and he treats me really well. He spends most of his time with me despite a 2 hour distance and is very respectful and trustworthy. I know he would never ever cheat on me. I love him wholeheartedly but we do have some different hobbies. He is extremely pionate about cars, and while I think cars are pretty cool, I'm not as into them as he is. Lately though, he has been messaging this girl alot on fb who is also really into cars and I can't help but feel a little intimidated. All they really do is talk about cars, and he has mentioned me in conversations, but I still feel like the messaging has gone a little overboard... Am I overreacting? I need some honest opinions please? Thank you