Twitter confidentially submitted an application with the U.S. Securities and Exchange Commission for a planned initial public offering. The fact that Twitter was able to apply confidentially means the company made less than $1 billion in revenue in the most recent fiscal year. 

Twitter is just over 7 years old, so why an IPO now? Here is a brief timeline of some of the major milestones for Twitter that have led up to the decision from the San Francisco-based microblogging service to go public.