They wouldn't. And if they lied to you, that's fraud, and you can press charges against them.
If they don't lie, but they don't tell you good things about their platform (you'll have to judge the amount of 'spin' on your own)...then move on. It's really that simple.
From your example, you could contact Facebook and ask for some stats about their application platform (telling them that your company is thinking about building an app for that platform). If they refuse to give you stats, either ask to be transferred to a 'higher-up', or tell them you're sorry you couldn't work together and get off the phone. If they do give you stats, you need to judge for yourself if building your app will be profitable on that platform.
There is absolutely no company (or person) in the world that is going to be able to quote you a hard number on the amount of profit your app is going to make before it exists.