It feels so presumptuous to ask strangers to join your fire team, so I just played most of the campaign on my own. Sometimes I'd run into people who were around my level and appeared to be playing the same mission as me, and I'd be, like, "yay, friends", but then we'd go through a door and they'd dissipate into thin air for no apparent reason.
I can see how you'd get more out of the game if you were into all the multiplayer stuff, but as someone who was just playing this for the campaign, I was very underwhelmed. I guess I was expecting this game to have varied missions? Or at least a story to speak of. Joke's on me, I suppose.
But I can't complain too much since I was buying a PS4 anyway, so Destiny essentially cost me £1.5. :p