BTC is not, itself, a security. However, there are people creating and buying and selling items packaged as securities in which the underlying business is BTC-based. They trade on what are purported to be "stock exchanges." Some of these are outright Ponzi scams.
Similarly, BTC is not, itself, a Ponzi scam. However, Ponzi scams exist in which the underlying currency is BTC.
If nothing else, some of these highly questionable products look like some kind of commodities future.
Fair point. It is possible that a btc-denominated contract could be considered a "security" especially since many of the promoters refer to them that way. I consider the bitcoin "stock" exchanges to be amusing novelties at the moment, but for sure some of the larger issues do involve material sums.