Range doesn't make something a public forum. Generally, control of the space by the public (or their delegates) does.
A letter to the editor of the New York Times has the potential to be seen by 343,000 households (a number down from 1.3 million). This never made the New York Times' letters page a public forum.
Twitter, for example, was at least previously accountable to shareholders. Now, it's very much a private forum.