What is social networking site? Is piktalk.com a ...

Social networking sites are sites where you can grow your social network with friends, colleagues and family etc. These sites allow you to share what ever you think is considerable for others. You network contacts can see your activities and vice versa.

If you a topic about any thing gets popular through social network sites it will be called social media optimization.

Some of giants in social media are facebook, twitter, orkut, myspace, linkedin, stumblupon, digg, delicious

From wikipedia

"A social networking website is one that focuses on building online communities of people who share interests and/or activities, or who are interested in exploring the interests and activities of others. Most social network services are web based and provide a variety of ways for users to interact, such as e-mail and instant messaging services."

Size doesn't decide whether or not it is considered a social networking site. Facebook and MySpace are the big ones, but there are literally thousands of smaller social networking platforms out there. The site you were asking about "Piktalk" is a good example of a smaller niche social networking site.