What the US government really wants is all your public online accounts.
They used the broad and not fully defined definition of the word to their advantage.
What a social media site is, is something I think we are all collectively defining. It's too new to be really set in stone.
Also, even though words have broad meanings, it doesn't make it right to use it that way, since social customs would make the person using such definitions unintelligible. For example, if I said in my introduction at a party to a woman "I'm gay", she will likely take it to mean 'I'm homosexual', not 'I'm happy'. Both are completely valid dictionary uses. Only one has a valid use in any modern context.
Social media is generally defined as media where the primary intent is socializing. For me, that means it is more about communicating with friends and family in a manner that is directed to furthering my understanding of them, and their understanding of me. These sites would be like facebook, twitter, instagram, etc
Sites like HN, webmasterworld, blackhatworld, etc... are more about advancing knowlledge in a specific area. Socializing is a secondary consequence. These could be considered 'quasi' social media accounts.
Based on the current definition of social media, even comments on a news site or blog, 'could' qualify. But then again we run into the issue that if you use words in non-standard ways you can become unintelligible to your fellow humans, undermining the whole purpose and concept of language. If I said 'I read a comment on social media' and really meant 'I read a comment on cnn.com' people would not understand what I'm saying. It would be odd and awkward.
So would you say that Reddit isn't a social network? I would have agreed at inception, but not anymore. I'd say the lines have blurred enough to look at any site where commenting is one of the primary functions as "social".
It's really not.