What does it mean for a site to be a classed as Social Media? To me it's pretty simple:
With those elements in mind, lets see how if popular social media sites mesh with them:
Facebook— Users can create content through posts or status updates, which can be commented, or favourited by other users. Users can friend other users which gives them updates and alerts about the activities of each other.
Tumblr— Users can create content by creating posts on their own page, which can be reblogged and commented upon, or liked by other users. Users can follow each other to get alerted.
Now lets pick a site, one that the majority of people wouldn't consider being a social media site— The New York Times and see how it fairs.
New York Times— Users (editors, journalists) can create content by posting
news articles and editorials. Users (website visitors) can comment on or share
that content with each other.
User's can follow (via RSS or Email alerts) some categories or journalists.
I point this out because the foundations of most websites are quite similar— we store content, present it to users who can often comment or share that content. The social element is essentially just breaking down the barrier saying 'this set of users can create content' and allowing everyone to create that primary content.
These common foundations are great news, because we as developers can learn how to build these foundations to get our sites ninety-percent complete, then add in the special sauce that makes our web-application unique. For Youtube, that's video, Facebook has a friendship confirmation, Twitter has their hashtag for discovering trends.
Visit your favourite sites, but step back and think about the foundations, scrape away all the pretty interface and think to yourself: what is the primary content? What is the site's idea of comments/likes? How do users interact and find each other?