Once your key is generated you can click the 'copy' icon to the right of your name in the header. After that you can share the key on any platform you like, including Keybase.io :)
Please stop referring to non publicly open platforms as they were actually usable.
There is keys.gnupg.net, pool.sks-keyservers.net, pgp.mit.edu, etc. These are the well-known ones that had been around for a while.
Should note that I never use the thing since in practice it's easier to fetch my key via traditional methods, a la pgp.mit.edu...