The typical process works like this:
1. Brand creates facebook page
2. People who really, really love the brand connect with the page.
3. The brand observes that people who have connected with them on social media are excellent customers.
4. The brand tries to increase the number of connected users by offering promotions and similar.
5. The new connections do not have the same value as the originals because they are attracted only by the offers and promotions. Which the brand then needs to keep offering otherwise "engagement" and other similar metrics drop off.
6. The original connections, the brands best and most enthusiastic customers, are now trained to only purchase when a discount is offered. Margins suffer all round.
It can appear that social media offers an easy way to create revenue. The long term effects are much harder to measure (because there is a huge lag between action and measurement) but they are often negative.
Social's value is seeing how people react to your brand, absorbing their feedback, connecting directly... but the planners have reduced it to numbers, like they do everything, and sucked the life out it.
However, the main way I've seen people do this so far is with viral content. Producing something that can go viral is very difficult for some brands - their culture does not support the risky, balls out attitude that does well online.
(If you have examples of viral content that does not fit this pattern then please let me know)
Other short form work also tends to go viral, YouTube is often credited with rejuvenating SNL, despite NBCs insistence on taking down every clip
People are brands too. I almost only watch talk show interviews in clip form.
What people don't want to do is to be constantly bombarded by bullshit communications from said brands. In many ways, openly declaring your brand preferences is signaling - the "liking" of brands is for their friends, not an enthusiastic wish to receive "special offers" from you. What they dislike even more are the highly intrusive ways brands are marketing themselves on Facebook.
It's great that you're running a promotion for a brand I like. No, I will not let you post to my wall as me. No, I will not spam 15 of my friends with your stupid offer before actually receiving said promo. No, I don't want to be forced to "Like" your stupid brand before I can even look at your page.
Brands, I want to receive pertinent information from you. If Starbucks is opening a new store in my area, I totally want to hear about it. What I don't want to hear are bullshit "news stories" concocted by your marketing department that is entirely content-free and screams like a cheap attempt to keep your name in my head.