Web Events doesn't support ads measurement at this time. Median session time and total time spent reports are not available yet. These features will be added in the future.
If you're already using the Facebook Pixel, you'll instantly start seeing web activity data in Facebook Analytics without making any changes. You'll also be able to use all of the features of Facebook Analytics, such as creating funnels to measure conversion, viewing cohorts to measure changes over time, and using demographics and segments to better understand particular audiences. For example, segments allow you to see the demographics of your top 10% of users or compare how one demographic group converts in your funnel at a different rate than others.
In order to get the most out of Facebook Analytics, you should trigger the page view event on all pages in your website. This will ensure that you're getting accurate user counts and session length time. If the page view event isn't being triggered on all pages of your website, then your active user numbers and session times will only measure activity on the pages where the page view event is being triggered.
See the Facebook Pixel documentation to learn more about how to use the Facebook Pixel.