Interesting stuff.
I think they somewhat misused that 20% statistic, though. I followed that link and it seems like the problem it refers to is tracking down organization-internal info that isn't in a searchable format (hence the suggestion of org-internal social media). People need wikis. ; )
There's also this:
Although the total size of the text corpus is large, each team’s corpus is relatively small and thus allows us to devote more computational resources to each message during ranking.
That sounds kinda like "might not scale to big Wikipedias", alas. Bummer. They also use a lot of features that aren't applicable to us, unfortunately. But the relevance vs recency thing is indeed thought-provoking.
Thanks for sharing, Erik—much appreciated!
—Trey