I have installed SharePoint SP1 on my development machine. Yammer and OneDrive links have appeared in the Central Administration: If you go ahead and click “Configure Yammer”, you can activate it: Because:

Yammer is Microsoft’s recommended tool for social collaboration.

When you activate Yammer, you’ll get this dialog, and the Yammer link in the SuiteBar: What happens when you click on Yammer, is that you are redirected to Yammer.com and you are prompted a login page. Then you have a usual yammer site with all your networks and stuff (in my case, SPC14 network): Then, if you go to your newsfeed, the following message is shown: