It has to do with interchangeable goods. Typically, only interchangeable goods are in competition. I.e. if I want paper towels, I can buy Brawny or I can buy the store brand. I may have a preference towards one or the other based on price or performance or something else, but if the store is out of one, I'll just buy the other and move on with life.

A petroleum based car is largely interchangeable with an electric car, assuming we're talking one that will probably comply with environmental regulations over it's lifetime. I might prefer an electric car because of the environment, or to support the movement, or whatever, but at the end of the day, a petroleum car still gets me where I'm going. Tesla is unlikely to become a monopoly because even in the electric vehicle space, there are interchangeable goods. I'm not intimately aware, but it sounds like there are a couple other companies that make competitive models.

Where that interchangeability can get weird and not so clear is on a more specific market, where users don't necessarily have a choice. Tesla is the only company (afaik) that makes a fully electric truck. You could possibly argue that Tesla has a monopoly on fully electric trucks; I think the question becomes, are other goods interchangeable? Is a petroleum truck interchangeable? Is a fully electric SUV interchangeable?

Applied to social media, each of the major social media networks offers or encourages a substantially different type of social interaction. Twitter is largely for piecemeal content, and is largely more public than other forms of interaction. It leads to really high levels of engagement, and lots of flame wars. Instagram is all about photos, people go for the glamour. Facebook attempts to make you engage with your network more, I find people share more personal information there. Reddit is more anonymous than the other two, and builds around the concept of communities, which are featured more prominently than the other platforms.

I think we all agree nobody has a monopoly on social media. The question is whether it's possible to have a monopoly on a particular form of social media. Are Reddit and Instagram interchangeable for you? They aren't for me, so I would say that they aren't in competition and as such, the existence of Reddit doesn't prevent Instagram having a monopoly any more than the existence of Chiquita does.

"Social media" is an incredibly diverse category of services. Deciding the monopoly status of a company based on the health of competition in social media is like deciding whether to break up Standard Oil based on the health of the entire raw materials goods sector. It's not a granular enough measure, because it contains several non-interchangeable goods. If Standard Oil jacks up the price of oil, I can't just go buy iron instead; I can't put steel in gas tank. Likewise, if I get pissed off at Facebook and decide to quit, I can't just go somewhere else. My 80 year old grandma is on Facebook, teaching her to use Twitter is going to be a problem, and I generally don't know if I want to expose my grandma to the cesspool that Twitter can sometimes be. The services are not interchangeable to me, so Facebook has a monopoly on that service. My choices are to play by their rules, or to just bow out of the experience entirely. Let's say we ignore the legal technicalities of a monopoly for a moment; doesn't the outcome look remarkably similar? If this doesn't count a monopoly, it seems to lead to the same place, and perhaps it's non-monopoly status is due to a flaw in the law, rather than being expected behavior.