SSL cert & FB Tab

Manny,
No problems for the questions. A self-signed certificate will not work with Facebook. Anyone can self sign a certificate, so while they can secure communication (when at a coffee house or something), they aren't trusted. Because of that, Facebook doesn't allow them because they can be forged for 'bad' things.

As for using one cert for many sites, that's not possible. An SSL certificate is always tied to a domain. There are 'wildcard' certificates which will work for all subdomains for a particular domain, but I've never heard of a certificate that works for more than one domain. Even if you found one, my guess is that it would be very expensive and cheaper to simply buy a $10-12 / year one for multiple domains.

In general, the 'cheap' certs are just fine for Facebook. We really recommend making sure that you always redirect the www version of your domain to non-www (or the other way around) so there's only 1 way to reach your site. That's good for many reasons, but also makes it so you don't have to buy a certificate that works for both www and non-www (which are cheaper).

Hope that helps,
Alex