I\'m building facebook App, but as I understood I need SSL.
Is there any way to avoid buying SSL?
You can avoid using an SSL certificate under these conditions:
https://example.com
You can avoid buying such certificate if your hosting provider has one for you (for example: heroku - ssl setup).
You can try to search for free certificates. As far as I know, http://startssl.com provides such a service.
You can use self-signed certificates. Your users will be prompted to accept validity of the certificate (or forced to add exception, which is a cumbersome process).