Like what? Just curious.

till they throw you the curveball that they can indeed talk to someone on twitter from facebook.

This is exactly like email though.

You have a gmail account that is tied to google. You have to login to gmail to access your email but you can email anyone in the world. Some people use different providers so they have different email addresses.

If you want to change providers there is no easy way to do it. You can use imapsync or export to pst and import to new provider and so on, or maybe your new provider gives you tools for importing mail from your old mailbox but it’s not a feature of email protocol(s) to do this.