Facebook will not be the largest Social Network by the 31st December 2030, as measured by Monthly Active users

For sure, I'd bet that will happen even sooner

Facebook is already seen by the young people as an older generation's social media platform. Coupled with all the frequent criticism it is receives from the public at large, on a number of recurring topics, Facebook faces a non-trivial battle to remain relevant. Having said that, its deep pockets, dominant market position and the startup culture in some of its departments may prove it resilient.

There are many different companies attacking niche groups that are pulling people away from the larger social networks, while scandals around Facebook's boycott and Twitter's celebrities hack increase the suspicion around these larger networks.
One particular rising contestant, Mastodon, is exposing a new way of having an online presence, closer to the blogs we experienced a long time ago. The way Mastodon works is that each "institution" can host their own network on their server, similar to twitter, so your company can host and control everything on it. The key insight that allows Mastodon to grow is the protocols to message across instances, so that groups aren't siloed in their own systems. This provides the network effects and connectivity that we look for in a social network, while allowing each user to delegate control of their servers to an institution they trust, instead of being forced to trust Twitter / Facebook / another behemoth. Personally, I am much happier delegating this control status to my company, my University, my political party or another institution better aligned with my personal values.