It was good while it lasted.Twitter announced on Tuesday that it's expanding its ad offerings so that brands like American Express can send promotions not only to users who follow their accounts but also "to mobile users that share similar interests with their existing followers." Brands will even be able to target the ads based on the type of device so that "the users who are likely to purchase their products" will see the ads. We shouldn't be too surprised that Twitter is going down the mobile ad route. It was less than three weeks ago that Facebook rolled out a similar program that allowed brands to push messages to users that Liked the brand. Twitter's going a step further and offering up their entire user base is aggressive. Then again, Twitter might as well. It isn't making much money as it is.