A number of my friends are fairly serious amateur photographers: people who have built up a repertoire of knowledge, various sorts of gear, and who display photography publicly online. Photography is certainly an excellent pastime. It satisfies geeky cravings for toys to play with, while serving as a creative outlet. It also lets you document and share what is going on in your life, with a group of friends who are increasingly likely to be far-flung (as we stay in touch with friends from former schools and employers, all over the world).
In addition to those appealing elements, photography has an interesting role within group dynamics. Everyone wants flattering photos of themselves, so being able to provide them makes you valuable to others. There is also competition between people who take photos. It takes place on the basis of quality of output, creativity, photographing interesting things, and gear. Indeed, photo gear is an increasingly appropriate way of demonstrating wealth. Whereas in some social circumstances, automobiles are probably the premier form of wealth expression, that isn’t well matched to a lifestyle where people move around often and relatively rarely see their friends in person. Photography is useful, visible, and a way of demonstrating capability, access, and wealth.
[Aside] On a somewhat related note, OKCupid has some data on what makes an attractive photo. Specifically, a non-flash shot taken with an SLR or 4:3 system camera at f/1.2 or f/1.8. The average 30 year old iPhone user has also had significantly more sexual partners than the average BlackBerry and (especially) Android user.