Article Popularity keeps track of activity on U2 Exit and calculates a popularity percentage for each post and page.
The popularity display you see:
Popularity: 27% [?]
is the rank of that post or page compared to the other posts and pages on the site.
Since page view counts, etc. are cumulative, newer posts will generally have lower popularity values than older posts. This, of course, evens out over time.
The values for different events (page views, feed views, comments, etc.) are configurable on a per-site basis, so each site may rank their content differently. This useful feature comes courtesy of Alex King.