For flirt and dating site
Some people are referred to has social butterflies basically based on how social and flirt they are. Using a .SOCIAL domain name for your flirting or dating site will surely fit into the modern belief of not being tied down to a single partner and freedom of choice in sexuality. Also since almost everyone Want to be considered social then it will be a perfect name for your website.
For specialized community website
Website that serves as a meeting point for a particular set of people connected based on their profession, beliefs, origin etc. Could also enjoy the use of a .SOCIAL extension. Since its basically just and online version of a get together, where people socialize with each other.
For social networks
With the success of social networks and apps like facebook, twitter, bbm etc the industry is expanding extensively. But that doesn't mean there is no room for new players to come into the game. Using a .SOCIAL ending for you social network is just another way of being different from the norm. And being unique will always count in your favour in most cases. So don't waste time, just get started with your dreams social media.