In all the time I've been an entrepreneur (over a decade), I've built many successful businesses, sold my companies, etc.. I've never had the problem of anyone stealing my idea except on one incident. And that guy didn't even understand the nature of the business fully or where the vision ended up and gave up after a few months.
Let's be clear about a few things. Your company WILL pivot whether in a small way or big way. Your overall mission and vision WILL change. Anyone that thinks they got everything figured out from day one deserves to be called a retard. Not telling people about your idea has more negative consequences than sharing it with the world.
I can't keep track of the number of times I've received valuable insights, advice, help, connections, etc etc... telling people (including strangers) about what I'm doing. The feedback and help I received in return help me make valuable pivots as needed. Thinking you should hide your idea means you should really look closely in the mirror and reevaluate yourself.
While I believe there are some things you should keep secret, the general idea itself is not one of them.
Side note: Those who use Facebook as an example, please be realistic. Had the Winklevoss Twin and their friend continue, do you realistically believe Facebook (by any other name) would be where it is today? Most of the later innovation that kept Facebook the leader it is today is base on the current Facebook team, not some concept that the idea originated from.