Having read all of the questions and answers (1 2 3 and so on) on here relating to DNS load balancing, and Round-robin DNS, there's still a number of unanswered questions..

Large companies, and I'm looking at Google, Facebook and Twitter here, do present multiple A records.
1) If DNS loadbalancing/failover is so dodgy, why do large organisations do it?

There seems to be very little mention of "DNS Pinning", despite this (PDF) paper about it.
2) Why is DNS Pinning so seldom mentioned?

3) Are there any concrete examples of which ISPs and so on actually do rewrite DNS TTLs?

That said, I'm not entirely backing the side for using DNS for failover or any form of load balancing. For most networks, BGP diverse routing still seems to be a better fit.

DNS rears it's ugly head again. :(