This sounds like the same futuristic material coating the super-fast spaceship that Zaphod and Ford Prefect were admiring outside Milliways in Hitch Hiker's Guide to the Galaxy.
There are a lot of moving parts, but these are separate sub-systems with well defined boundaries of interaction. Each sub-system is implemented using technologies selected for specific characteristics.
Mono for long-running and complex backend processes.
nginx, PHP, MySQL, memcache - for presentation - quick!
node.js provides an event bus between PHP and Mono
MongoDB is taking over from storing large blobs of JSON in MySQL.
There is tons of job tweets, but most have "job" in the username and/or app source. Also any app source that contains "bot" or "feed" is most likely safe to ignore.
It's many pronged approach. User timelines are loaded for "reputable" users, (see blog for computing reputation) and search input is used with primative bayesian filtering.
In my experience, I have found Tweet source (web, Tweetdeck, API etc.) to be one of the most effective quality/authenticity indicators.
This sounds like the same futuristic material coating the super-fast spaceship that Zaphod and Ford Prefect were admiring outside Milliways in Hitch Hiker's Guide to the Galaxy.