It is currently Sunday here, and I posted the web application today on Facebook, and not during work, but my boss just notice it and felt paranoid.
Slightly above average or slightly below average?
Maybe you work in Zimbabwe or something, but that's way below average for a programmer even in relatively low-pay countries like Romania and India (I have worked with companies in both and personally know the people involved).
Don't confuse average salaries in general with the average salaries in your industry!
The job sucks? So what. Do it excellently anyway and work on your side projects and dreams of escape without calling undue attention to those things. Keep a low profile. Don't give your Facebook link to your boss. If he has it, don't post your side projects on it. Don't post anything anywhere that your boss would likely have an easy time finding it. Especially don't do so in a fashion that could be interpreted as bragging or which could threaten the company in some manner.
Not sure if something is "dangerous" to say/do? Then play it safe until you are sure how to handle it. When in doubt, err on the side of caution -- at least until you have a few months salary saved up and it won't be a big deal to suddenly lose your job. At which point, if a little bragging/sharing leads you to be jobless, you can philosophically take it as a hint from the universe that it's time to pursue this seriously as a business venture. It's a lot harder to take that philosophical attitude when you fear 'starvation'.
Good luck sorting this out.