The problem here is that your definition of the product is far too coarse. You can only really talk about competition for products which are truly equivalent. For an extreme example of why too coarse a definition of the product might yield a bad understanding of monopolies. Say that there was a food company and a water company. You might argue that both of these companies are competing in the space of "products that you need to live", but I clearly need both things, so sales do not form a zero sum game between the two companies.

So why is facebook a monopoly? For the simple reason that having access to your friends on facebook and having access to your friends on tiktok is not a zero sum game. There is a limited amount of tradeoff between the two, but it is expected for consumers to want both, creating intrinsic opportunities for the two companies to cooperate, instead of compete.

The problem is that competition is measured more or less as how anti-correlated demand for two products is, or approximately, How much less price sensitive my demand for the "sum" of the products is than for just one of them. If this is your measure, then you see that facebook and Tiktok don't compete all that much.

One easy way to fix some of these problems is by requiring something like XMPP. Peering between IM providers would make these things a lot more fungible. Google abandoned XMPP, hwich was very popular, for their inferior chat application, for a reason, and the only reason that I own any Apple products is because of iMessage.