I'll just dip in here to source something useful from one of my previous discussions on this matter from Eric. https://forum.pso2.com/topic/6141/managing-an-alliance-is-a-nightmare-a-leader-s-pov/25?page=2 this is just in regards to those looking to run alliances.
But anyhow, I honestly feel activity tracking is... interesting as there's not too much to work with in tracking it considering the last online is just blank. I'm actually putting forward a plan with mine shortly to try and recover the increase in inactive players over time. Honestly recruitment is like a day job. I generally find the activity is focused on a small number of larger alliances particularly outside ship 2. Generally, the two things to look for are pn names which most likely indicate a inactive player if longer term. The other is simply, just be open and honest about it, if you know who's who, and they're not about in-game for a while, let them know you need to focus your members on those playing but they're welcome to interact and return if they retake interest in the game, this at least leaves them the open door.
I'm not into enforcing activity posting from members personally as I believe they should enjoy it at their leisure. Just some thoughts though.