Uh...
First, you seem to think that search is a "gold mine". This is completely wrong. Ads are gold mine, but nobody is making much money off of search. Nobody is looking to "pay real money" to have their search queries answered.
Second, you seem to think that Bing is making money. They're not; the most recently available figures show them burning a billion dollars a quarter. That's not doing "well" in most people's books.
Third, a couple of minor points: DDG has their own crawler, and doesn't just repackage Bing's results. On the other hand, Yahoo does just repackage Bing's results, and is shutting down their crawler. I'm not sure why that makes Yahoo "second tier" but DDG a "weird niche search engine". :)
What Paul Graham realized (after Yahoo neglected search and paid a huge price) and Yahoo didn't is that people searching are looking for answers; and people looking for answers often pay real money to have their problems solved. That's why Google makes more than (say) Facebook.
The point is - search is a gold mine. The first tier (Google) is a mint. The second tiers (Yahoo, Bing, Baidu, Yandex) do well, unless they are weighed down with unprofitable divisions, or bad business decisions. Even a third tier search provider can make enough to be very viable. Third tier social networks? Even without the network effect, I wouldn't want to be there.