Facebook certainly wasn't the first website with a sidebar on the left or even right side. Just like YouTube wasn't the first to have that kind of color scheme. It's definitely not a bad idea taking tricks that big corporations use and making use of it on your own. So unless someone is stealing another's property I don't see any harm in it.
It's very likely Facebook & YouTube take ideas from other sources too.
Fully agree. Inspiration is a powerful thing, and taking concepts
from other themes to use as inspiration
is fine in my honest opinion.
Where I don't
agree with, is with people blatantly just ripping themes and content.