Standard Oil was a monopoly for over 20 years. There are countless monopolies that have gone on that long or longer, only to be broken up by government eventually.
I love the free market, but between regulatory capture, legal costs, and aggressive pricing structures, monopolies can be absolutely unbeatable within the career lifetime of any person.
No one except Zuckerberg, I'd think.
However, it raises an interesting question: how exactly would you break up an entity like facebook? It doesn't seem like a business that would lend itself to subdivision.
Then mandate interoperability with an open standard, like a phone network.
I don't think that makes much sense for breaking up something like Facebook.
Unlike a phone network, Facebook's service is not rooted in geography. Yes, it's users have homes and geographic locations, but those are just arbitrary account attributes, just like their name or if they like the Denver Broncos. You might as well have one post-breakup company start out with all the A-M accounts and the other the N-Z accounts.
If you had to divide up the users between N successor companies, I'd probably be best to do it based on friend-graph connectivity, with mandated interoperability between them. That might make the successors sticky enough to survive.
So your premise is true(ish), but the conclusion can't be drawn from it (and your comparison falls apart) because we don't need strict dependency on geography, only a (relatively strong) correlation.
That way each competitor could offer its own interface and algorithm for filtering people's feeds, and show their own ads (or offer a competing business model). If users opted to have their posts on the competing site automatically sent to Facebook (so their Facebook-only friends could still see them) then Facebook would be free to data-mine those posts still.
The next question is, could someone code a browser extension or other tool which makes this possible without needing any changes from Facebook's side? If it would break the Facebook terms of service, would someone crowd-fund the anti-trust lawsuit to override those terms?
Hell, the main benefit I see to instagram is that their geotagging improved immensely.
Network effects. Together everyone hates it; individually, there is nowhere else for people to go without losing the majority of your social network. Facebook has people; its software is utterly irrelevant to why people use it.
But it seems many aspects of capitalism lend themselves to winner take all mechanics, resulting in large companies (airliners and telecoms and entertainment companies) constantly merging and reducing competition. So without the FTC, we might have even more monopolies. Without these laws, Microsoft would be the only PC company (MS bailed apple out to keep some "competition" around) and Intel would be the only manufacturer of CPUs (likewise for Intel and AMD).
"Natural monopolies arise where the largest supplier in an industry, often the first supplier in a market, has an overwhelming cost advantage over other actual or potential competitors; this tends to be the case in industries where fixed costs predominate, creating economies of scale that are large in relation to the size of the market, as is the case in water and electricity services. The fixed cost of constructing a competing transmission network is so high, and the marginal cost of transmission for the incumbent so low, that it effectively bars potential competitors from the monopolist's market, acting as an early insurmountable barrier to entry into the market place."
There's a good argument to be made here they all software sub-markets are natural monopolies.
But we'll never know, since nobody else can reasonably enter the market.