Regardless of the veracity or not of Compete stats, I personally completely removed /. from my daily visited sites about 2 months ago, after 10+ years of it being my #1 morning destination.
I have nothing tangible to pin this on - they haven't done the digg-style algorithm implosion; their recent redesign wasn't a gawker-esque breakage which made it unnavigable. It's more subjective, but here are some of the points I can find which I think contribute to my decision, in no particular order:

The 1-2 day lag has increasingly become an impediment to the value of the discussion. There have been comments on /. for a few years such as 'I read this 2 days ago on X'. 4-5 years back there were few viable alternatives for quality discussion where those experienced in the field participated, so this could be tolerated. HN, Twitter and to a lesser extent reddit and specialist forums have meant the discussion has often already happened, and for /. it's more recap on topics hand-selected by the 'editors'...

...against whom the vitriol has been rising. While 10 years back the sources were varied, /. has increasingly become 2-day old metadiscussion on whatever was getting pageviews on the 'tech' linkbait sites - TC, Gizmodo, etc. This has trended, AOL-way style away from real tech and more towards shiny/flamey/'socialmedia-y' puff-pieces. This is a fundamental problem with not trusting your community to select the stories, but relying on hand-chosen entries. Not just the lag, but it's too open to skewing/editorial bias/corporate control. Idle was an early indicator that the corporate overlords were after this, and even ghettoising it there didn't stop the creep of topic away from the more hardcore tech which had initially attracted...

...the greybeards. The dotcom boom created a lot of techs. Many had no experience, and were looking to the older hackers for advice. Now that 'boomer' generation is 10+ years in, and have been developing successful enterprise and their own communities, they have considerably more to offer. These communities have also learned from some of the flaws of /. - as mentioned the hand-selection, but also...

...+1 funny. This is critical. While sometimes HN can appear a little dry, the actively enforced rule against one-liner witticism forces those of us who are given to facetiousness (myself included) a reason to pause and hopefully construct something more thoughtful. This was always an issue with /. - you never got Karma for funny, which led to the crazy 'insightful' mods to reward humour. This gaming of the comment system IMO devalued it.

So that's a few things off the top of my head. The primary reason for me leaving wasn't these per se, but more that I found increasingly that I would visit and not see interesting headlines, or would read a thread and find nothing but snarky jibes, meta-discussion about /. and the editors, etc. I felt this degradation of quality exponentially rose in the second half of 2010, and so around Feb this year I completely stopped going. I am sure I'm not the only one and this sort of decline is self-perpetuating.

I'm not happy about it - I credit /. with a huge amount of my versatility these days as a tech, and broadening my horizons on a great many issues. I see HN in some proto-stages of this decline at the moment, and I'm hoping the control structure here will allow the community be self-aware enough and proactive enough to take this experience and make some tweaks to avoid the negative trends that growth in readership/participation bring to all online communities (Usenet, /., Reddit have all seen this happen).

Old stories. Less insightful than HN. Lame.