HN user
Definitely backlinks, number of referring IPs/domains is very big indicator of website Authority.
Besides that, UX factors like bounce rate, session duration, etc. are very good indicators as well.
Hello, one of the developers here. What you said is not quite accurate, as our ranking algorithm will still rate websites with high popularity/traffic (Alexa Rank) even with no facebook shares / referring domains or any other metrics. They will not reach such high positions, but they will be ranked.
MEGA! it's end-to-end encrypted and 50GB free! they already have desktop client for windows and will also release for Linux and OSX shortly.