Facebook was able to avoid the scaling problems that plagued Friendster and still plague Twitter by gracefully dropping information. Your news feed may be behind some of the time and may occasionally be incomplete. Most people don't notice or care.
Facebook initially had a fairly detailed security model and fairly cautious default settings. This became a problem because it led to significant computational overhead serving requests.
Even an extremely well funded firm like Facebook realizes that if the 97% of photos that nobody cares are public require a db lookup, that's a lot of extra cycles. Due to the nature of the social graph, the problem was increasing exponentially every day and had to be dealt with.
So Zuckerberg decided to just cut away a lot of the added load in one fell swoop by changing the default setting on a lot of stuff. The servers and accountants breathed a sigh of relief.
The fallout has been remarkably small considering how significant a change this was and how crippling it could have been.
I imagine the like button announcement was intended to distract from the security changes, but instead it sort of added to the concerns.
It will blow over. Scaling problems aside I think the main advantage Twitter has over Facebook is that it doesn't try to include much of a security model which means everyone understands the security model. Nothing is really private on Facebook anyway.