I saw a several year old site suffer the same hit starting June 2. By June 4th, we lost 80% of more of Google referred traffic.
Pure speculation makes it look like a bounce-rate/page thing to me... the site still ranks well for 2-5 word (head and longtail) phrases with low bounce rates, but pages that were very popular with higher bounce rates seem to be suffering the most. (Or maybe it's just amplified due to the page volume)
Some of the visitor traffic has returned (almost like the index is learning as it's being rebuilt) but still down 35-40% from Mar-Apr levels on the same pages/terms.
Again... purely speculative... but I have been taking steps to increase visitor experience.
- Moved it to a faster, dedicated server.
- Working on reducing bounce on pages/phrase matches that still do well.
- Looking at the pages that used to do well, and reducing bounce on them as well.
As far as links go... I have never really had a link plan for this site and all have just come naturally. That said, its not thousands of links either!