Well, LinkedIn is the niche while Facebook is an all encompassing network. For B2B purposes, LinkedIn is both more effective and easier to use. You can certainly do this on Facebook, but you have to do a better job of sifting through non-sense and developing worthwhile contacts.
For professional point of view Lindedin is the best social networking site to search and find a job. For friends and promotions, Facebook is the best to explore..
I think every online business should have accounts at both Twitter and Facebook. Both target B2C communication. However, they're a bit different. Facebook is for open communication between companies and customers, when all real names are revealed, when websites promoted are absolutely serious and A-list. These days potential customers would go to the company's FB wall directy to see if there are complains, how the company handles support, what customers are happy about. This is fake-free feedback. Twitter is more shady in fact. It doesn't mean that only crappy businesses do that, but Twitter means less open communication. It might be good for some reasons.
LinkedIn is more of a a professional network frequented by companies, managers, recruiting staff for reaching out to their employees/future employees/prospects. Facebook is a social network that helps people stay in touch and also for companies to reach out to their customers getting their feedback, engaging them in a friend activity, etc.
As you mentioned, Twitter, LinkedIn and Facebook offer very different things, thus I have accounts on all three because they are able to meet three individual and separate needs. G+ on the other hand, has not differentiated itself. I don't see what the benefit is to having a Google account the way I see how LinkedIn can be beneficial. I believe this is the biggest problem with G+ and will be why it doesn't succeed.
Facebook definitely has more users I think it depends on the product you are trying to market. Linkedin is definitely better for marketing SEO software and things of that nature but Facebook all the way when it comes to physical items
^ Agreed facebook does have more users and almost everyone on linkedin has a facebook account as well. But like i said above depends on what you are trying to market. Also does anyone know how much it costs to have a game script that i bought integrated to facebook? Since we are on the facebook subject? If it is free that would be amazing! LOL, but i doubt it is.
I think facebook is the best for it has a lot of user and a lot og regular user that online daily. Additionally it has a lot of application.
Both are good social networking sites... Linkedin is professional site so facebook having more users...