Being able to measure influence or activity on Twitter is a really big problem.
Brands spend lots of money, in form of hours, interacting with people on Twitter. Of course, they like to have something to show for that investment so there is a need to measure that activity and determine some type of ROI.
I'm not going to get into the whole ROI of social media debate but I'm going to say this right now:
Measuring activity and influence on Twitter is hard because it's just like trying to measure how somebody works a room at a networking event.
I believe that it is best to use a combination of quantitative and qualitative data.
Looking at number of followers and or following can be misleading.
Klout score can be misleading.
Reach-type numbers can be misleading.