Sorry to burst your bubble. It is highly likely that at least 2 of the members in this site know each other.
I know it was probably in some just but the 8 billion you quoted includes people who don't use the internet (e.g. babies, those without computers/internet access and technophobes).
Then you also have to disregard pretty much anyone who can't read or write any English (except the really persistent folks here who exclusively use translators).
Then you have to factor out most of the population who aren't suicidal, the places where the site isn't visible or isn't that well known.
Etc. Etc.
So that 8 billion number is significantly smaller. Plus as the website gets more popularity then more people are going to use it - those who are suicidal and those who are worried about someone they know who is and others trying to find dirt on the site itself.
Also, correct me if I'm wrong, but most posts are visible without a login right? So people could see the posts and then think it sounds like someone they know, and then join.
So the moral is, I'm not trying to alarm anyone but don't give away too much info if you don't want to be found on here.