Twitter took a big step forward last March with its self-service ad platform that was designed for personal branding and social media marketing. And a year later, after much feedback, Twitter is ending its invite-only service and opening ad-buying up to all U.S. users. This means that anybody can now sign up and buy promoted Tweets of their own, with all the same sorts of analytics and marketing tools provided that the big advertisers and campaigns get with their promoted tweets. This doesn’t mean that you’ll see more ads on the social media service per se, but you might start to see some more unexpected promoted tweets pop up as users begin to experiment with the ad-buying service for their own individual projects.