Red flag or no?

There’s this girl that my boyfriend has on Snapchat. She snapchats him every day and I’ve seen a few and it’s just streaks and stuff

But he doesn’t have her on Facebook? She has sent him a request on Facebook but he hasn’t accepted. I don’t know the chick and shes only got 1 mutual friend with me on Facebook and it’s not one of my boyfriend friends

They follow eachother on instagram

Does that sound odd?

Like I completely trust him and it’s not about that, I just don’t know if this sounds like a red flag or not?