They may not technically be the only platforms (extending to include all big tech), but practically speaking they are effectively the only platforms, which is what really matters. Someone may technically be able to change from Comcast to Verizon, but if it would cost them 10k then it becomes rather impractical and the net effect is the same as if Comcast was the only option. The barrier to switching must be considered as well as the existence of options.

However I agree that breaking them up may not be the solution - mandated interoperability may be. At least then there would be the benefits of having freedom of choice with plenty of options without the downsides of losing access to your data and your social network (which I think constitute the majority of the switching barrier). Similar mandates in healthcare and education would also be beneficial. This would of course not immediately solve the problem, but I would rather have to deal with getting developers to be mindful of the needs of their users than face the impossibility of getting users in the first place.

An entity should be allowed to be as successful as they can manage, so long as the practical barriers to competition are not affected (ie, network effects)