Player retention is a general term for how likely a new player will stay and continue playing P&W after a certain amount of time, and for some free-to-play game developers what really matters is "first day player retention", or how many players are willing to continue playing after their first daily session. While it's harder to measure player retention accurately, activity is strongly related to player retention.
Now if you look at P&W, a little more than 45% have been inactive for more than a week on average, while less than a third of players are committed enough to log on daily. And Sketchy's stats show that a majority of nations aren't even in alliances. This doesn't speak well of P&W's activity, and its player retention is likely below average (search up recently created nations just one week from now and tally up how much are in blue/green/yellow compared to orange/red/purple) as far as F2P games go.