There’s been a drop in participation through contributions - post rate, answers per question, and relatively active users (5+ posts per week) have all been trending downward for a while. post 
Since June of this year we’ve seen a major jump in traffic - nearly 100% - due to changes to Google’s page ranking, with an accompanying jump in participation through community moderation (mainly close votes and down votes) ... but, crucially, not an increased volume of questions over the same time frame; they’ve essentially stayed flat.
There are a lot of valid arguments on why this is happening, but I think we can all agree that this is not a great direction to be going in.
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The author goes on to discuss several hypothesis as to what is going wrong. I picked up on this conversation when it spilled over into the Wiki-Research-L mailing list. posts
One observation new to me was called the Facebook Effect. This means that as more people devote their internet time to that one service they loose interest or remain ignorant of other opportunities for participation.
Andrew Lih
I wish we had the slides for this, but Jack Herrick of WikiHow presented at Wikimania 2012 on the features put in to promote more community growth.
There is video, however! And the exact time code is here: video
Quick transcription of that section of Jack Herrick's talk:
Jack Herrick
"And sadly this problem seems to be getting worse. This is the famous chart, I'm sure a lot of Wikipedians have seen this chart... the blue line is the active wiki editors. It's been declining... it peaked in 2007. It's been declining or flat ever since. And that graph is more or less the same at WikiHow. We peaked in 2007... I have some theories about why 2007 was the magic year for wikis, but I won't go into that here..... I wasn't happy with those results, so I spent the last three years trying to turn that around."
Jack: People leave WikiHow not because they don't like it, but because they've found somewhere more fun...
video
Theories as to what happened in 2007 - Seigenthaler event and press. Facebook came in 2008... "and we weren't keeping up"
video