Photo sharing is the top reason people use social networks. I don't think there's a close second.

I suspect being able to send a message to a friend without having to remember/maintain phone numbers or e-mail addresses is also a key feature for social networks. It provides a reliable way of finding someone, and it means people can maintain their own real-life contact details in one place and have everyone else's records update automatically and accurately.

But yes, sharing photos is surely one of the most important applications for such a platform. People like to keep in touch, and in the age of cameraphones and high speed Internet connections, a picture often is worth 1,000 words.

There aren't many messages more effective than "X has just tagged you in a photo" to drive a user back to a site.