Every post you make actually gets you another "point"! There is a little counter beneath your current rank and one that shows up when you click on your name in the right corner. Mine says "48 points until my next rank."
Getting a follow from someone also gets you a point! Both posting and gaining followers will contribute to you getting closer to your next rank. There are some forum sections though that state posts there won't help your rank up.
Edit: Here's what I'm talking about. Post count won't increase in the "Ask a Pony" section, so if you're looking to level up fast, posting there won't help. D:
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Igloostellar Snow's post in How to Get Points to Level Up was marked as the answer