Imagine you get 3,000 posts per month free. That's an average of 4 posts per hour every 24 hour period (6-8 more likely, because sleep). Most users won't come close to that, but businesses likely will depending on their usage and nature. You could even go the wireless phone route where you buy a subscription of messages in blocks, like 6,000 or 10,000 etc. This allows for monetization without punishing those who are experimenting or casually using the service.
Price-per-x forces users to think about how much x they use. This is only a good idea if your users (a) need x or (b) are already hooked on x (say, by a couple of years of free x).
If there's a "Like" or "retweet" equivalent you might also get extra posts for that (like you get more credits to post questions on Quora when people upvote your answers or follow your questions).