Ask Ubuntu and the Ubuntu Single Sign On and two different things.
Ask Ubuntu is owned and hosted by Stack Exchange. While the Single Sign On(SSO), is owned by Canonical.
Being the creators of Ubuntu, I can be fairly certain Canonical are running Ubuntu Servers. And most likely using the OpenSSL library.
Again, I can be confident that they had patched and fixed the bug. But, if you are in doubt, it's never a bad idea to change your password.
P.S. Since you are using the SSO to sign in to Ask Ubuntu, your password is never shared with Ask Ubuntu (nor any other site whatsoever).