Yes, it is very natural - human nature is like this.
Some people accept new things with enthusiasm, but after some time they loose their commitment. Some groups will continue growing and gathering relevant content, some will fall into oblivion and despair.
What can you do about it? Of course, you can ignore it - keep your photos in groups poor weak management, or even leave this groups.
Personally, I am active only in small subset of groups where I am member, my participation in others is only passive with photos posted on request.