A new study shows that having obese friends increases a person's odds of being obese. In this way, obesity could be thought of as contagious via a person's social network. This doesn't seem too far-fetched that one person's eating and exercise habits could affect a friend's.
While all this seems plausible (and probable), I would put forth another idea. Last year, a study was done that showed how oral bacteria may influence obesity. Researchers were able to identify 98% of overweight subjects by alterations in their oral bacteria. Given that, isn't it possible that friends could end up inadvertenly sharing their oral bacteria?
I'm sure if friends are eating together that oral bacteria could be transferred from one person to another. Whether that's enough bacteria to change a person's existing oral bacteria is debatable. But other conditions are spread through kissing, so there is a plausible mechanism.
Maybe this idea will get studied more in the future. Until then, it's just some speculation on my part.