There are a lot of companies that have a company facebook or linkedin page. Many have a corporate twitter account, or more than one. This is a good thing and those companies are likely doing good with their social presence.
The problem with this approach is it really doesn't scale that well. Imaging a company with 1000 employees. If they have one or two people focused on social media, that's good. They can probably build a few thousand followers, have regular blog posts and updates on their corporate pages.
If they spent that time and effort on training the 1000 employees on the proper use of social media, encouraged their employees to tweet, or post photos on pinterest or talk about work on their facebook pages or in their linkedin updates then instead of reaching a few thousand people, now you can reach a few hundred thousand people.
I recently had a linkedin update turn into a really good sales lead. The interesting thing was I am not the social media representative of my company. In fact I'm not in sales, or marketing. I do though update my linkedin connections with relevant updates from my company and one of those is what prompted the lead.
Now there are some risks which is why you need to train and educate people to avoid some of the "twitter disasters" that regularly make the media. You also need to have a great company culture and employee morale. Luckily we have both but if employees hate your company or aren't engaged then this is either not going to provide much value, or can cause brand damage.Though if you have unhappy employees you really need to fix that regardless of social media.
Leveraging the entire company to help market and brand your compay and products can really help increase awareness and revenue. It's not without risks, but all rewards come with risks.