If you build it, and it's genuinely, uniquely useful, you won't be able to stop people from coming and inviting others to join.

Telling everyone about it is only necessary if you're indistinguishable from 100 different takes on the same thing, and trying to win a shouting match (this includes hurrying to shout the world down before competitors get a chance).

But as the saying goes, you are not in traffic - you are the traffic. The reason you need to shout is because of people like you shouting.