>
> One thing is for sure YOU can't login FACEBOOK with two different IDs
> from the SAME PC at the same TIME.

I would assume that to be true for using une WEB-Browser. It shpild be
no Problem to login with two IDs using e.g. chrome for one and foirefox
for the other and there is nothing preventing you to login with an third
ID using konqueror.

> There should be a UNIQUE field such MAC ADDRESS + IP + ID managed by
> the DATABASE in a TABLE !!!

That would suprise me. They certainly do not manage such an table for
current logins.

Client does have an cookie to identify. This cookie is known at
application server which stores such information in RAM. Keyed by that
cookie there is more information about user session stored in RAM.

You should read some basic information about how to make WEB-Applications.

Elmar