Earlier quoted context omitted.
To call HN a social network is to redefine the term to apply to any website that allows comments. It's really not.
That's how the US Government defines it. If you want a US Visa you need to tell them your HK account.
The DF-160 form lists a set of social networks, not otherwise accessable that I've found, here:
https://ceac.state.gov/GenNIV/General/complete/complete_cont...
Included are: Ask.fm, Douban, Flikr, Google+, Facebook, Instagram, LinkedIn, MySpace, Pinterest, Qzone (QQ), Redit, Sina Waibo, Tencent Weibo, Tumblr, Twitter, Twoo, Vine, Vkontakte (VK), Youko, YouTube, and "none".
Hacker News is conspicuously missing.