You're killing the game.
I have found little information on Guild activity on this forum, blue posts and around the net. From what I gather, Guild Activity is a point for point comparison to the amount an individual contributed or would have contributed to the Guild XP. There are more than likely other contributors to the final number and feel free to post on this if I'm missing a key atribute or two but either way, the final Number does absolutely nothing for guilds.
What can we use this number for? I've asked this question to a lot in WoW trade, general, and other guilds, and all I get is 'Well, you can see who's contributing the most.' What use is that to a Raiding guild or a PvP guild?
This feature would be amazing If you could either personally adjust this from guild to guild to focus their goals. I get the programming side of things and I know there is more to it then wishful thinking but what it seems like they did was develop the requirements and code for guild XP and just made those calculations visible individually based on character. This is NOT Guild Activity. At least, This is not what you lead most of the WoW community to believe when announced.
Beyond the numerous other issues Blizzard has had with this launch, this probably is highest on my list for faulty game development.
How to Fix this:
Distinguish Different types of Guild Contribution: Raiding, Pvping, Crafting, Dailies and Individual Contributions. Allow these to be viewed in the roster as One drop down option: Guild Activity, with said types as columns. If you feel that isn't enough, Maybe Allow further control by the guild to customize what they feel is considered guild Activity. And finally, Instead of All time and Weekly options (weekly is useless for anything but guild xp contribution) allow at least a monthly view as well.
P.S. A left and right arrow button would be a great addition to scroll through the Roster drop downs in the Guild U.I.