this is quite an easy problem. it takes a short time to integrate twitter as an option, and then you can promote the idea on your site, and see if people switch, and how many people use them in conjunction. then if time doesn't tell you on it's own, if considerable number of people are using both, figure out some polite way to have them do a survey to scope out which one they use more.<p>in this way, assessing the risk of a complete migration will be possible, and you can make a more intelligent decision than you can today. something that disruptive could easily alienate your user base, and winning people back is perhaps even harder than winning them over the first time.<p>don't forget that twitter is unproven, there is ongoing speculation about if it is still a fad, and whether it can keep the average person's attention for longer than a few months. to bank on it against an established technology like SMS is too risky without more information. if twitter changes the landscape, the testing model will allow you to ride the wave, and if it doesn't, then you're still around when twitter fades.