Yammer Hammers Forward With API Launch; See It Soon In Twhirl

Yammer, a Twitter-like messaging system for businesses, has seen solid growth since launching last week at TechCrunch50 (and taking the top prize).

CEO David Sacks says there are now 10,000 networks and 50,000 users just one week in. Yammer’s business model is to let people use the service for free, spreading it throughout the enterprise. When and if a company wants to take administrative control over the account, Yammer charges $1/user/month. Administrators can set access controls, such as IP controls and SSL.

The company already allows interaction with the service via the site, an AIR client, iPhone, Blackberry, IM, SMS and email. This evening they’ve also launched an API to allow third party developers to build Yammer into their applications.

First up is Seesmic, which is simultaneously announcing that they’re buildiing Yammer support into their Twhirl desktop client. Twhirl is used today to access the Twitter, Friendfeed and Seesmic video chat services. Integration should be completed in the coming weeks. (Disclosure: I’m an investor in Seesmic)

We signed up to Yammer just after the TechCrunch50 conference last week. Nearly everyone is using it regularly now. We have 14 members and 161 total messages.

The Yammer demo video from TechCrunch50 is here.