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.