Here's the feature list:

• Threaded Conversations - Easily follow conversations with replies displayed underneath the original message.
• Private Messaging - Communicate privately with one or more colleagues.
• Read/Unread Message Tracking - Identify previously loaded versus newly loaded messages and replies.
• Full Retina Display Support - Experience enhanced text, images, and more.
• Multi Feed Access - Quickly navigate between group feeds, company feeds, and @mentions to find relevant conversations.
• Upgraded In-App Browser - Seamlessly navigate between Yammer and links to external sites.
• Document Viewing - Instantly access Microsoft Office documents, PDFs, and images appearing in a feed.

Yammer plans to release new versions of its mobile app for Android, BlackBerry and Windows Mobile later this year. Each of these platforms already has an app, and a mobile optimized website and SMS integration are also available.

Yammer also has a desktop application, or can be used with Seesmic or SharePoint.

Yammer competes with many other enterprise microblogging and social networking services.