Joan Rivers -- Promotes iPhone 6 Feet Under

The ghost of Joan Rivers has a message for you ... BUY THE iPHONE 6!!! After all, who knows more about the cloud than a ghost?

This post appeared -- briefly -- on Joan's Facebook page Friday morning ... clearly the result of a pre-negotiated deal with Apple. Someone pulled a Ron Popeil ... set it and forget it.

The post was removed pronto.