What do you mean, "Who knows?" in terms of reach? That's why I have separate accounts for my separate brands. If I had one account, it would depend on the business type (i.e. who my clients are, retail vs. company's or publications).
Re: FB - I'm using that just for retail (weddings), and I don't need to generate a lot of traffic to be sucessful - i.e. too much could be a detriment. I want to shoot 20-25 weddings a year, and to do that, need to generate about 200 leads. If I was reaching more people, it might result in fewer qualified leads and more time sifting through folks who are not my clients. For retail, my best business has always come from word of mouth, and I spend my retail marketing time there.
For b2b though, I've found adwords to be fantastic but you definitely have to spend time curating it and staying on top of it in order to make it work.