tedster - 9:01 pm on Jun 23, 2010 (gmt 0)
For several years I've noticed some rankings that flicker, or "yo-yo" as we've also called it here. In at least a few cases, it seemed to be related to backlink profiles that look a bit "too good to be true" in some ways, but they don't show enough natural variety or different types of backlinks.
In those cases (and I've seen this hit some major companies) it seems to be a filter that gets triggered when the backlinks, even if very good, still look too much bit like "artificial buzz" to Google.
You mentioned social media in your post. Are those social media mentions helping the word to spread about your site? If so, that kind of obviously authentic link can sometimes get the filter removed over time.