Downvotes don't mean censorship. You clearly made your point with complete freedom of speech. Agreeing with you is another thing.
Good question. I think it does if downvotes result in inability to express an opinion (as opposed to simply having downvotes shown).
If downvotes are used to suppress freedoms then downvoting is a means of censorship. In other words, there is no big difference between prohibiting something by one person (dictatorship) or by 10 persons (collective dictatorship) (if it is not prohibited by law of course).