More than 32 million people have registered with the social network that has one rule: tell the universe what you’re doing in 140 characters or less. It used to be that you had to text Twitter (40404) to use this versatile service while mobile. But now there are Twitter apps across the most popular smart phone platforms that make the service much more convenient, allowing you to use multiple accounts, include your location using GPS, and even add links to music clips, pictures, and videos.