How about monetize search.twitter.com like Google does with its search? How about providing analytical tools for business, such as monitoring their brand and sentiment? How about providing businesses with user profiles on likes/dislikes, i.e. potential customers?
What's interesting is that other businesses have already started making money off the data Twitter is providing, like http://whatthetrend.com/. It would make sense for Twitter to monetize their immense data and boost revenue, although that would probably kill these competing businesses.
They'll figure it out.