Will someone steal my idea if I use Balloonary?

The short answer is: NO. More explicitly, you might be afraid that someone could steal your idea if you announce it publicly. After all, your idea is amazing enough so that anybody would fall in love with it on first sight, right? And letting a precious idea roam freely is any entrepreneur’s worst nightmare, isn’t it?
Nope… it’s the worst nightmare of anyone who has never actually made a big idea come to life.
Ideas are cheap, execution is expensive. Most ideas have little to no intrinsic value, and usually no one will steal it.

What VCs invest in is not the idea but rather the team and the accomplishments of the execution so far. A minimum viable product that shows some real world market traction is worth gold.
So to summarize: Be bold, get out there, and grow with the feedback the world has for your idea.

Here’s one exception though: If your idea is not just an idea, but rather a patentable technical innovation, you should probably not disclose your blueprints on the internet.