Matchmakers are usually trade secrets, but even the best ones are worthless in games where taking an objective gives an advantage.
MMs exist to give teams equal odds of winning, but every lost objective makes the odds exponentially harder. Your teams may be evenly skilled, but if you lost the first 10 or 15 minutes of trading, it's very likely you can't recover -- unless your team is more skilled, in which case you probably weren't snowballed so soon. In games like World of Tanks, League, Chess, etc, a working matchmaker doesn't stop snowballing; in fact it often enables it.
I like this article about snowballing because League is particularly prone to it. I haven't played all that long, but I do already see some power creep with new champions; they're often designed to counter a popular meta... Which inevitably leads to a scenario like the old lady who swallowed the fly.
The League matchmaker seems to grab players on a 1-1 basis. If you queue, it finds somebody with similar skill and puts them in the other team. It doesn't seem care too much about how you compare to anybody except your match. So it isn't uncommon for me to be in a lvl 20-30 game, but each team has 2 <lvl 10 players.
It also seems to normalize very quickly; if you have an uncharacteristically high winrate for the last few dozen games, you quickly move into matches where you're probably not prepared for the level of skill. I have a lot of losing streaks after I've played well and they are honestly frustrating. I don't think Riot will ever tell exactly how the MM works, but it's not the major problem. Objectives are just too rewarding and losing 2 or 3 isn't usually salvageable. Having good laners on the team seems to be the only factor in swinging the match; minion waves are easy to clear and freeze enemy progress.
