Last time I checked, the site-specific container extensions (Facebook and Google) had a "don't open anything except the target site in the container" feature that appeared to be missing from the multi-account containers extension.

I agree that the best solution would be to add that functionality to the base extension rather than create a specific extension for every site people might want to block.

My understanding is they kick in automatically. So even if you're on someblog.com, the FB 'Like' buttons won't be tracking your behavior, nor will Google analytics and Google Doubleclick.