I don't think LinkedIn is good for this at all. I see things like earnings press releases on LinkedIn, or "XYZ, Inc. invents new 2nm transistor that won't be around for 300 years." on LinkedIn. LinkedIn presents itself as a social network for professionals, and I think they cover that quite well.
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 :-)