Basically Akamai is using the same ssl certifate on (most?) of its edge servers. The reason for that is that traditionally it is difficult to decide for a server that is serving multiple domains, which SSL certificate to show for a client -- the HTTP header, which contains the hostname is sent way after the certificate information has been exchanged.

A certificate can contain several hostnames (in the SubjectAlternativeName extension) - but that does not scale if you have a big number of sites for a number of reasons (re-signing the certificate all the time is a nuisance, browser behavior with certificates containing several thousand hostnames is kind of fun, etc.).

Nowadays there are solutions to that problem (using the Server Name Indication TLS extension -- which basically sends the desired hostname in the TLS exchange before the certificate is exchanged). However, the number of sites actively using SNI is very low - google is the only site known to me that is doing it (try accessing google.com with/without SNI and you will get completely different certificates).

The reason why SNI is not yet that much used is that client support is still a bit flaky. Afaik it is supported by all recent desktop browsers. However, I think the XP TLS stack does not support it (and there are still enough users on that), android only supports it starting with version 3.0, etc.

So - at the moment you basically still need a separate IP for each site (or at least one IP for sites that can share one certificate).

I don't know if akamai also supports custom SSL certificates. Facebook seems to use kind of an interesting mix between akamai and self-hosting - facebook.com itself seems to be hosted by facebook. However, if you use facebook over ssl and check the url of served profile pictures, you will see that they go to https://fbcdn-profile-a.akamaihd.net (or similar) -- hence to one of the hostnames that is mentioned in the akamai edge certificates.