My site apparantly feel out of favor in the past several months, being left off its main keyphrase search (must now enter ='.www') but G also picked up on the linked Twitter account that was started early Oct.

Could this be due to lack up updates, lack of backlinks, a combination thereof or possibly even visitors not spending enough time per visit?

It used to come up #5 to #7 for the search before the last six months or so, then looking back further, it would even come up #1 or #2.

Personally I beleive G will be adding more weight to sites having established/engaging FB | Twit accounts. But not going too far --- otherwise they'll dilute their own strength/branding via their indirect endorsing of.