Overall, Posterous has 9.2 million monthly visitors on its own site and on custom domains, according to Quantcast, which measures the service directly with its permission. That’s up from 6 million in September, but still quite a bit less than competitors like Tumblr and WordPress, which have 59.6 million and 517 million people, respectively.
The new groups product sends a ton of email (with user permission, and with the help of SendGrid): 230,000 messages per day. Half of Posterous group distribution is over email rather than the Web, and 30 percent of users are not registered.
As I mentioned earlier, most groups–76 percent–are private. And a quarter are for corporate groups–an alternative to business collaboration tools like Yammer. Business groups have an average of 15 people, while family groups have about 10.