I think approximately 300 people nearly died and I'm the only one who knows
Why would you be the only one that knows 300 people died? Are you involved in something illegal?
No, but I'm thinking of posting it on my wall.
If you know about people dying you should tell the police. Posting on your wall is not a good way to let people who can do something about the situation know.
I don't want to be annoying and make a big deal out of it.
If someone is in danger, you should always call the police. Not reporting it makes you complicit in whatever happens, if not legally then morally. You shouldn't be flippant about people's lives.