And while valuable in it's own right has no where near the engagement numbers of Facebook.
Twitter doesn't quite fit nsns's description as a 'Companies Only' social network, as much of it is social. I don't necessarily know that anyone would willingly sign up for a companies only social network, despite what companies may dream and pray for :-)
LinkedIn is mostly about connecting professionals to each other and companies
Facebook (in a business context), is mostly about connecting brands with consumers.
Consumers are not professionals, although the underlying people that participate in those roles may largely overlap (if you think about it as a Venn diagram)