Matchmaking aganst friends

I'm just wondering what is the chance for the matchmaking system to place a friend in my friendlist against me, giving that we're in queue at almost the same time and that our elo aren't much different. Because yesterday I had an interesting match where I was going Support and my friend on the enemy team is going ADC, both of us just soloing but somehow ended up in the same match. I didn't check what elo the others are in, but I am unranked and he/she is Silver IV, so I don't even know how accurate the matchmaking system is (sometimes it just sucks).

(Seriously though, I felt like I am in one of those pro streams that got match up against other pros since the Challenger elo is quite small. It's just nice to play against someone you know in a solo game lol)