Jack Dorsey
Vivian Giang / Business Insider
When Twitter stated its $26 price per share yesterday, co-founder Jack Dorsey was expected to make a lot of money. 
He'd have made north of $600 million at that price, but he wouldn't have been a billionaire from Twitter alone.

Now, Twitter is trading at $45.
So Jack Dorsey and his 23,411,350 shares are now worth about $1.05 billion. If Square, his other company, goes public next year, his billionaire status will be solidified.
UPDATE: As Forbes correctly points out, Dorsey has technically been a billionaire since 2012 between his involvement in both Square and Twitter. That would require him to sell his Square stock on a secondary market though. Now, from Twitter alone, he's reached billionaire status.
Congrats!