What explains the explosion in social photosharing entrepreneurial activity?

Believe it or not, I think that this has less to do with photos and more to do with building a social platform.

Why?

Sharing photos is a base activity of the "social" pyramid.

Whether you are looking at Facebook or Orkut, etc., it is one of the most popular activities that users engage in on social networks.

So, if you are going to start building a new "social" platform, you want to start with one of the most popular activities...like photo sharing.

Once users have populated the site, then you can expand into other activities, but initially you want to offer them something that they use/need.

It seems to me like these entrepreneurs have wisely cherry-picked one of the most common activities (photo sharing) in hopes of attracting users and later leveraging the user base to add additional features, ex. social commerce, etc. 

So, for example, if Path gains a large number of users, one can imagine access to a user's Path social graph being more valuable than their Facebook social graph.

Why?

Because the user's Path social graph is populated by their "real" or close friends, as opposed to their Facebook social graph which contains a wider web of acquaintances.

Assuming that the social graph will be used for social commerce, the Path social graph will be way more valuable than the Facebook social graph because the trust ratio (used to overcome counter-party risk--arguably the greatest impediment to e-commerce) will be much higher.