Co-op missions

I'm starting to experience something where I'm starting to get stuck on. I'm currently at a mission where you need to do 3 challenges in co-op. But I don't have any friends to invite and also nobody ever want to join the invite you sent them. Got a feeling that this one quest is gonna take forever. The idea of co-op is nice but don't force it into missions like this, buzzkill after some time...