Twitter began the roll-out of its long-awaited self-serve ad platform with electronic payments enabled -- a cornerstone of its revenue model -- in mid-November with a group of fewer than 20 advertisers.

A self-serve tool has been planned since Twitter unveiled promoted tweets with six advertisers in April 2010. But the company has been methodical in rolling out advertising products and has grown its customer base of advertisers buying promoted tweets, accounts and trends slowly.

There are 2,400 advertisers on Twitter, split between the direct sales team, which handles Fortune 250 company accounts, and the inside sales team, focused on small- and midsize businesses, which comprise major global brands as well as firms with just a handful of employees.