I think a lot just depends on where you are at in the hobby with regards to what you consider activity (relevant posts to your skill set). I always recommend the site for people new to aquariums or getting back in after many years. Plenty of knowledgeable people on various topics on subject matter but it isn't the site I recommend if someone has a more in-depth interest in reef tanks, planted aquariums, LED
I find the site to be active with regards to numbers participating but much of the activity is always more social related in my opinion and not really in-depth aquarium related. How you like my new light? What did you do in the tank today? Look at my new fish? Many topics are geared more towards social activity and don't often lead to "meaty" discussions on topics one will learn something from.
I don't see drops with holidays, time zones, etc.. much in my opinion. Smart phones, tablets, etc..are so common place any more most have instant access to sites and drop in multiple times a day. I do think because of that fewer post because they take in so many sites and topics, it becomes more about just keeping updated/following along sort of thing.
Even topics that are fish related won't pull in numbers. Let me put up a tank picture with clown puke gravel, sponge bob ornaments, and a few live plants stuffed into the gravel and title it My Planted tank and you have eliminated many that are into planted tanks that would love to discuss dosing, lighting, plant selection, etc.. They may comment nice looking tank, off to a good start, etc.. to be cordial but they aren't going to really get involved in the thread to make it one of save to favorites tabs. We all have them those links to huge threads chopped full of information, ideas, etc..
I also see a very large/diverse group of people on the site. This brings a lot of different takes on matters but also stymies some discussions. It is interesting to follow discussions between a middle school/high school individual, one in college, older individuals, etc... They often are directed very much on where one is at in life with regards to what one wants in a tank. Some are very much into the biological and chemical processes others just want cool fish it often doesn't lend itself to active threads.
The best cure is just to post and create topics to keep you interested and draw people in. Also can just contact individuals about topics and get side discussion through email, chat, etc.. going also.