It seems to me that a social network is one of those natural monopolies. It works so well because so many people use it. Remember the big reason many social network alternatives failed, no one else was on them. There are some things (very few in fact) that just work better as monopolies. But that just means you need to regulate it differently. And there's an interesting discussion, how best to regulate natural monopo…
The better question is, why would you need or want to regulate it as a natural monopoly? Specifically: to accomplish what?
Let me give you some examples.
1) To force FB to open up, allowing other social networks to openly ride on its network / social data (the broadband one pipe lots of delivery companies premise). Ok, now you're begging for a radical increase in abuse of personal data. And you're going to need some new (or expanded existing) bureaucracy agency to manage it all, which will open up new government abuses without a doubt (happens every time; and said agency will radically slow down innovation, which also happens every single time). Some obnoxious SNEA - Social Network Enforcement Agency - will get created, and that'll be the end of any innovation in social media; if you give the Feds an inch of new power, they'll take a thousand miles.
Besides, you can already replicate your social network onto other platforms by allowing FB apps to access your list of friends. Most people don't care because there simply are not that many highly compelling social concepts to explore that are worth the effort to maintain/use. There will never be large numbers of compelling social offerings; there inherently can't be as people have finite time, the hurdle to acquire it is very high. It's work/effort to maintain these networks for the end user, they do not want to have to upkeep it all.
2) To enforce higher standards on privacy. Well, we can already pass legislation to do that comprehensively across all media platforms, if it makes sense. It'd be ridiculous to regulate one platform for that purpose.
3) To limit various corporate abuses by FB (Instagram/Snapchat, ala Internet Explorer/Netscape). Well, we already have anti-trust laws for that and thousands of other laws & regulations and numerous giant three letter enforcement agencies.
Meanwhile, while the argument is being made for regulating Facebook as a natural monopoly, the next technology paradigm is being born somewhere to make them far less relevant. That process has been repeating itself for 60 years or so now, requiring very little actual regulation by the government.