1. Ability of web application that how long does it take people to complete the tasks for which they are on the web page.
2. Free from mistake that user often makes in any web application.
3. Good visibility of links/info that are useful for user.
4. Amount of traffic - how popular/user friendly the web application is.
3 Things:
1. The amount of traffic generated - Can be measured by any tool like Google Analytics
2. The time spent on a page or the web app - Can be measured by any tool like Google Analytics
3. The exit path or the Bounce rate - Again use an analytics tool to see from where and how the user is exiting
It's really hard to quantify something like usability. I can think of only one way i.e., by taking users' feedback. One can have a way of categorizing the feedback as good/not good/bad/ok or one can even allow users to rate the application. Then the feedback can be used to come up with a number to quantify usability.
The main drawback of above method is that it is very much limited to the kind of users who provide feedback. One has to make sure that feedback from all the targeted categories is represented in quantifying the usability factor.