So far it's been just a theory of mine. An interesting observation is that Facebook can easily deny this concept saying that "slow rollout can prevent excessive server stress", which is without a doubt true.
It seems to me that it is hard to test whether the random rollout just aims at preventing server crashes or it has behavioural connotations as well.
Yup. It can catch bugs. It also allows them to catch drops in numbers and other problems. This is the main reason that the 2013 newsfeed design never made it to anyone else.
Also, at least in the past, FB used to roll out redesigns to groups that were relatively isolated from the rest of the graph like New Zealand.
