SAN FRANCISCO (AP) -- Buying ads on Twitter is about to get easier for small businesses. That's because the online messaging service is adding a key piece to its moneymaking model.

Twitter is making it more convenient by unveiling a long-awaited automated system that will enable advertisers to manage their marketing campaigns and budgets without having to deal with sales reps. The self-service approach announced Thursday initially will only be available to advertisers who accept or use American Express cards before Twitter opens the system to all comers later this year.

Flipping the switch on self-service advertising is the latest sign of Twitter's ambition to create a powerful marketing vehicle in the mold of Google and Facebook.

It also marks another stepping stone toward an eventual initial public offering of stock from Twitter.