There have been (and continue to be) many. However, the first one that came to mind after reading your question is this:
In late 2010, I wrote a Hub about hunting for jobs in the Bakken oil formation area of western North Dakota. Google liked it, so it moved up to visibility in the search results and got a few daily views. So far, so good.
Then, perhaps 6-8 months ago, job seekers suddenly seemed to discover it en masse. It could be that major news stories on the Bakken had raised awareness and thus increased the number of searches. I don't know. But...magic happened.
Before long (after the "surge" started), readers began posting helpful information in the comments--places they'd found that were hiring, how to interview, housing challenges, whatever. As a result, it (the page) grew into a "community forum" with people helping people. More than 1,000 comments accrued, and I added a fresh "spillover page", which continues to grow rapidly.
Most importantly, people from across the country who were/are using the Hub as their primary info resource began actually getting HIRED, sometimes even meeting each other in North Dakota or at Job Fairs after chatting online for weeks or months.
THAT experience, I wouldn't change for the world.