Before FB there was MySpace, widely used and nobody could possibly compete with them. Then FB came along with a BETTER product.
MySpace didn't have much penetration in college students. FB started there. Now FB has penetration throughtout all demos. There is no thin-edge to exploit.
Being unable to attract customers is not a barrier to entry. Having to build your own railroad in order to sell oil is a barrier to entry.
The "product" FB sells its users is its other users. By definition, no other service provider can provide that.
You mean that if you stop using FB you have absolutely no means to communicate with your friends?
Yes. If I don't use FB, I don't get invited to events. There are other means of inviting me, but other people don't think of them.
Now, on raw communication, you are correct. I can be phoned, emailed, etc. And do.