There are several reasons for that:
- First (at least for me), an unsolicited e-mail goes directly into trash and I seek the unsubscribe button. Under no circumstances I am interested in additional stuff somebody has to offer beyond the first service.
- People who did not buy your stuff/service can still like you on Facebook, they can do it after they have forgotten some login to your site etc.
- The single best advertising that actually works is recommendation from friends. I think that seeing that somebody else liked something on your feed gets pretty close to that.
- People do like to have all of their "Internet" in one place. Why go to mail if they can have everything in their feed?
- If you use Facebook then they solve a huge number of stuff for you - administration, spam filtering, server hosting, account management (there are other solutions to this of course, but it is good to be able to throw all of this on somebody else)
- I think posting on media sites wastes no more time than creating mailing lists and also requires less tools.
- Advertising for Facebook is mutually beneficial, the more people there are, the more you can reach.
- Also, ads on Facebook have great reach (from what I have heard) and are quite cheap.
- I would hazard a guess that most people are more likely to have notifications on for Facebook rather than e-mail.
Facebook is also probably not going to fail, preparing for this scenario is a nice thought exercise but at some point you have to have some faith in your suppliers. Same goes for Google (mail, docs), Dropbox (file history), Github (issue tracker, code reviews).