It used to be that advertising on Twitter was limited to large companies and brands that could afford it and with whom the microblogging platform chose to work. But come late March, any business or brand — regardless of its size — can promote their product or service on Twitter by using a new self-service tool similar to the ones offered by Facebook and Google AdWords.

To kick things off, Twitter announced a partnership with American Express late last week to give the first 10,000 eligible Amex merchants and cardholders who sign up online initial access to the…