In Winter ’12, the following new Chatter objects have been added to the SOAP API (version 23.0).

• ChatterActivity : ChatterActivity represents the number of posts and comments made by a user and the number of comments and likes on posts received by the same user.
• ChatterConversation: Represents a private conversation in Chatter, consisting of messages that conversation members have sent or received.
• ChatterConversationMember: Represents a member of a private conversation in Chatter. A member has either sent messages to or received messages from other conversation participants.
• ChatterMessage: Represents a message sent as part of a private conversation in Chatter.

Here is a blog post describing these changes:

• New Chatter Objects in Winter ’12