The Post Frequency Rule
The frequency of posts in a service is inversely proportional to the size of the post. Said another way, the longer the post, the less frequently they will happen.
Take a look at stats from the three largest "default public" social media services:
WordPress – 430k posts per day
Tumblr – 31.8mm posts per day
Twitter – 140mm posts per day (march 2011)
Of course, these numbers are also impacted by the number of total accounts and active accounts on the system. None of the three companies post those numbers publicly. Based on the numbers I've seen, the ratio of monthly active accounts to total accounts is also highly correlated to the the size of the post. The shorter the required post in a service, the higher percent of total users will be active on it.
If you want to understand the power of Tumblr and Twitter, you need to look at how quick and how easy it is to post. There are of course many other factors at work, but brevity and ease is a big part of why these services work so well.