But it works, which is why they do it.
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.
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.
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.