As you're aware Twitter filed its public S-1 statement this afternoon. There's a great deal of interesting material in it. The company said that in 2010 revenue was roughly $28 million. Last year it was $317 million. This year it could well exceed $500 million, reflecting triple-digit ad revenue growth.
The following are the important mobile-related stats disclosed in the S-1 filing (mostly verbatim statements):
In 2010 74% of Twitter's revenue came from data licensing and the remainder from ads. In 2012 85% of revenue came from ads and 15% from data licensing, reflecting a huge shift in the sources of revenue for the company.
Given that Twitter has a still relatively small number of users in the US and internationally there's plenty of room for growth -- domestically and abroad.