I'll give you a counter-anecdote specifically about the competitive part: I'm currently about a year into the most consistent running habit I've maintained since I was running high school track years ago, and it all started and is occasionally sustained with Nike+ Run Club friend challenges.

I'm not running with any of the friends I compete against, we all live in different parts of the country. But on the days when it would be really easy to just skip the workout for some reason, not falling behind in that month's challenge is incentive to go do it anyways.

Could I have that kind of self control on my own? Sure. But I'm realistic enough to know that sometimes, a little social pressure goes a long way. Wanting to be the first person to hit N miles for the month (or just not be embarrassingly far behind the pack) is a very real motivating factor.

(But FWIW I only feel this way about running because it isn't my favorite athletic activity. When I spend a day throwing down 80+ miles on my bike, I don't use Strava or anything like that, the ride is the reward. But I can't do an 80+ mile ride on a weeknight after work for some quick cardio)