Legitimate decreases need to be considered, too.
Two effects that are common to all languages are stricter BLP policy and competition (for user attention) from Facebook. Both happened at around the same time (2006-2008) as the stagnation of activity. Whether this offers any explanation, I don't know.
Facebook might compete for people's time, but I don't think it substitutes for the reasons you edit Wikipedia.
For me I started out reading Wikipedia and had added a little content about my local suburb, but the motivations for those edits was curiousity, just to experiment with this Wikipedia thing. For the next couple of years, I just did little edits here and there, probably because I was annoyed with something I throught wrong or a bit incomplete. I had some of those edits deleted for reasons I never quite understood.
Then I read (via one of the donate banners) of the Jimmy Wales quote:
"Imagine a world in which every single person on the planet is given free access to the sum of all human knowledge. That's what we're doing."
And that changed my perspective of what Wikipedia was and what it was capable of. It was a "road to Damascus" moment for me. I became much more actively interested in writing new articles on topics that weren't there. Now my personal mission has been to try to collate information on missing topics and write at least a start class article.
I still don't necessarily like the way Wikipedia operates (technically or socially), but I'm probably more thicked-skinned about it. I'm here for a reason and I will persist because I am a "true believer" in the Wikipedia vision.
Perhaps we need to understand more about *why* people edit Wikipedia and therefore what will help them to become/stay active editors (assuming their "why"s are positive ones).