My guess is that they'll hope to gather traffic and addicts (Twitterholics) and then begin to add premium features for a fee of course.  Premium features? (enhanced handling of direct messages, greater granularity in filtering by language or region, etc.) 

However, they're presently in danger of loosing legitimate long-term users due to all of the spambots currently plaguing the system.  Also, people are tending to use the service as a chat system and not really answering the question "What are you doing?"

So, I expect eventually advertising or subscription fees will eventually appear once the owners feel that they've built a critical mass of users that they can reference as a basis for a fee structure.

 -- Mike