Forefront of technical challenges? Are you effin' kidding me? That website that can't get its shit together for 15+ years I use it. Constantly and consistently there are tons of bugs in its behavior. Doubling comments, missing pictures, can't create photo albums, then they are created twice, site unavailable, some parts of site not working, feed not loading, videos not playing, back button almost completely broken. I could go on and on and on. Any browser, anytime.
I've never ever experienced any other popular system/service so broken.
I am sure behind it all is complex overengineered cathedral of frameworks, systems and libraries. Some of it may be even cool. But the final result is piece of shit, no matter how I look on it, no matter how many chances I give it.
Compared to quality of say various google apps is... well incomparable. So yeah, having this on resume is underwhelming, even without looking at moral topics.