Let's say I have discovered that on an "about-to-go-booming" platform there isn't a social network client (A Facebook client for example). I can create one and submit to the platforms application store for distribution. What is the possibility for me to monetize on this besides putting an app as a paid one? Would for example Facebook bother if I run my own adds within this facebook application? Or am I even allowed to create a Facebook application without a direct approval from a trademark owner?
To create a Facebook app, you'll have to obtain API keys from Facebook which involves accepting TOS. It'll give you rights to use Facebook's contents. Its better to read its TOS to see what's allowed and what's not. Some trademarks are allowed and some aren't. Remember, you'll always be allowed to use a trademark (if allowed) to denote original service. You can't use it to name your own services/products.
And, you are free to put own ads in your app. But, you must check TOS (Its rare, but some may prohibit that).