I have been scrounging for articles/info about the architecture at Facebook, the challenges & ways they tackle them. What they use & why they use. How do they scale & what are the design decisions for what they do etc. Main underpinning being to learn. Knowing about sites which handles such massive traffic gives lots of pointers for architects etc. to keep in mind certain stuff while designing new sites. I am sharing what I found.
I have 2 more links but unable to post due to restrictions at this site. Also, please share if anyone has anything better (need not be related to Facebook only).
P.S. - I wasn't able to find good places to share this research, hence this initiative. Hope this helps someone.