A recent study of how sharing workout data influences people's fitness and achievements has been published.
Claiming that exercise is a "social contagion" and that including peers into your personal fitness regime is a huge motivator.
Interestingly, they also found that comparing ones own information to that of better runners wasn't as motivational as comparing data with less fit peers. Possibly showing the need to stay ahead of 'slower' runners to be more motivational than chasing quicker ones?
The study also showed that while men were motivated by both other men and women to exercise harder, women seemed only to be motivated by other women.