Some decline in activity over certain months is contributed by School starting or finals weeks.. mid terms.. etc. Summer is usually when the game activity is picked back up. At least, that's what i've noticed over the 5 years i've been here.
Just recently groups in game have been turning to private maps and chats as mentioned by Nephrite.. and might be a contribution of the decline in activity. But with this, we cannot be certain.
Around in-game staff held event times I notice activity picks up as well.. so maybe consider the months the seasonal events are held?
~ Whisper