
It's not so hard with Facebook to have multiple logins.
Unlike Google, Facebook does not have an ecosystem of websites with one unifying account.
OTOH, Facebook does let you post as a "page" in select places. If they really wanted to support multiple logins at once, they have a lot of the hooks needed to do it. But they don't want to. Supporting "posting as a page" suits a lot of reasons they think people would need to.
Wade.