Facebook has become THE way to simply justify the popularity of something. If you don't have a thousand "Likes" you're just not relevant. That's how a lot of people, including potential investors, see it.
I agree with you that Facebook is not the only way to gauge interest in something but since it's become the simplest, it's been adopted as the new "nielson box". People see a page has 10,000 likes and infer that means it actually has 100K people who like it! (Factoring in the people who would if they had fb accounts or people who have them but don't "like" pages).
And to further complicate things, it's not just the number of likes, but the amount of time it took to get them. The faster we can raise 1,000 likes on our page, the more of an impact it will have.
I agree that the system isn't perfect but I'm working with what I've got to prove relevance to the people who can make things happen that I alone cannot. If they told me to prove demand for something by starting an email campaign, you bet I'd be posting about sending emails!