The difference between Facebook and an ISP is that ISPs are, for all intents and purposes, a natural monopoly. There is an enormous cost associated with laying cable and negotiating the rights-of-way with the thousands of affected parcels to make even a tiny local network. It makes sense to enforce net neutrality because it's impossible to switch to a different ISP in many markets, just as it's impossible to switch to a different water provider or sewage system provider.
Facebook is a social media website. It doesn't provide a basic, necessary service. It doesn't operate under the physical constraints that utilities do, and it's not impossible to compete with Facebook, as evidenced by the numerous social media services out there like Snapchat, Tumblr, Youtube, Reddit, LinkedIn, Google+, Qzone (China), and Sina Weibo (China).
I'm certainly not against regulation as a general concept. The free market does not produce the best, ethical incentives in all cases. That being said, I fail to see why regulating Facebook is even necessary, and what that would look like in practice.
People on HN seem to take as a given that Facebook having loads of personal data is somehow a bad or dangerous thing. Why? Facebook has 2 billion users. That's 25% of the human population. 2 billion people voluntarily created profiles, provided details about themselves like their age and relationship status, and uploaded photos of themselves, their friends, and their families. For those 2 billion people, the "cost" of "losing privacy" was worth it to them. Face it, nobody outside of the tiny HN bubble cares about the security of their vacation photos or cat pics, nor should they.