I like this model, because then you have a scarce resource (albeit artificially so) that costs you nothing to make more of and you can use it to encourage users

I like this model, because then you have a scarce resource (albeit artificially so) that costs you nothing to make more of and you can use it to encourage users to do stuff you want, like inviting others. You could very well do it Dropbox style: you get 500 posts for every friend who signs up, and they get 500 too (starting out with 3500 in your example). It's not spamming if there's a gift. 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).