Users come and go all the time; sometimes you notice it and sometimes you don't. But its normal in any forum that life will get busy or slow and its the summer right now which is a high time when people go on holidays or spend more time outside where its warm and sunny and less indoors on the computer.
So its very normal to see a dip in activity of members because life just gets busy. We also tend to get new people turning up though because they've got a new camera or just got it out of the box because its bright and sunny and they've more time to actually take photos (even before or after work - whereas in winter its dark then).
Moderation side we've only had one ban as of late; however user in question has decided to try and return more than once which spawned repeat rebans so that might be something you've spotted.
A few others have chosen to move on for their own reasons. Whilst we try to please all and to make the site welcoming to all sometimes social friction at a personal level (often only between one or two members) sometimes means that people choose to move on. There's no harm in that, people have to choose where they like to socialise and if they find the social group isn't to their liking or has changed (or they in turn have changed) then moving on is sometimes the best course of action - even if many times its only a short term thing and we often see many people return in time.
Forums dip and swell in numbers naturally over time and TPF has a surprisingly healthy recruitment of new users (compared to many sites on the net these days). Facebook also has a part to play in diminishing user activity though TPF is weathering that very well