The slippery slope, as a fallacy, implies that two unrelated things, X and Y can lead from one to the other through a series of intermediary steps. Gay marriage is about consenting adults having the right to make the choice to marry as consenting adults. Therefore it is fallacious to draw a slippery slope comparison to it leading to necrophila as dead people cannot consent. This slippery slope is usually drawn by people who find homosexuality to be against their moral standards and hence they claim that tolerance of one 'immoral' thing is a slippery slope that will lead to other 'immoral' practices being tolerated. From our perspective, this is fallacious, because the argument is about the rights of consenting adults to live together and look after each other as they wish. From their perspective, they see, moral slippage.
A fallacy is only a fallacy when the conclusion is not supported by the premise. A slippery slope does not always have to be fallacious and does not automatically lose the argument--if it can be proven that all the intermediary steps link. In this case, the poster is only responding to the claim that since private companies do not have to respect our privacy rights, they can do anything they want with our data. This is of course incorrect because one form of communication is protected by law and the other one isn't. I'm not even sure if this is a slippery slope argument.
Both forms of communication are frequently owned by private companies, so one can't argue that private companies can do whatever they want. Private companies can only do what they are legally entitled to do. You might say this brings to light the question: if cell phone communications via private companies are protected, why aren't our chats and emails? And since we have privacy settings, or rather the illusion of privacy, it isn't exactly like it is easily made apparent that our private correspondence is anything but private.
It isn't a slippery slope to point out what happens when one's illusion of privacy is invaded. We've seen countless examples of this over the years.