I think user behaviour is not the main ranking factor. Easy, old forums that had been a great user metric suffered a lot from panda/penguin. If user metric would be a big ranking factor they should have been back to top. But instead of giving us deep content , pa-pe-goo gives us shallow blogs and spammy sites.
Sample two: sometimes whne pa-pe-goo has it good times, user metric on our sites are great, poeple fetching more than 6 sites in a row. But most time pa-pe-goo sends one-fetch-visitors, bounce is up.
If user metric would be a ranking factor than this should not be happening!