I don't think you want to try that argument with immigration officials, although it might just keep your incorrect answer from being straight up fraud or willful misrepresentation.
Knowing that, it's crystal clear HN falls strictly within that definition of "social media", although it might not be as clear if you don't know what that particular site is.
Googling 'site:gov "justpaste.it"' also brings endless results of government documents mentioning the site in the context of terrorism.
I somewhat doubt US immigration authorities thwarted any would-be terrorists by asking for their justpaste.it username, but what do I know, perhaps this was an important breakthrough in the global war on terror.
You'd be surprised at the number of people who willingly give up their social media accounts, only for immigration officials to find comments in support of terror attacks in the Middle East.
It's pretty easy to think it's harmless if you live in a country where that viewpoint is not uncommon.
It can be an easy charge of “lying to the government on an official form” when they discover you have a user account somewhere that you didn’t disclose, even if they can’t get anything else to stick.