Soap box

[Dave Paprocki]

Conventional attribute of street preacher, who would stand on soap box and say sermon, and thus, term denoting preacher-like behaviour.

[Mark Zuckerberg]

When someone gives an unimportant [impromptu] speech, gives their opinion in an unexpectedly long way, or goes on a short [tangent] about something; that the other person either did not need or want to hear.