A couple of major platforms like Facebook and Twitter, YouTube, have become in many places around the world a de facto public sphere. Especially in countries that have less than free Internet, less than free mass media. And these countries have transitioned from a very controlled public sphere to a commercially-run one like Facebook.
Where we meet the public matters. A public lecture delivered by a visiting academic in a university lecture theatre is very different to that same lecture being delivered in a genuine public space, be that virtual or actual.