According to potential acquisition suitors, Twitter is valued at $8-$10 billion. Acquisition talks are at low level by Google and Facebook executives and have gone nowhere. Twitter made $45 million in 2010 revenue, but lost money on hiring and running data centers. Twitter is expecting to hit $100 million in 2011 revenues.
When Twitter raised $200 million in their last round of funding, they were given a $3.7 billion valuation. Google and Facebook have both discussed buying out Twitter in the past. Apparently Twitter executives believe that they could eventually become a $100 billion business.