It probably used to be character limit, but again Twitter has already doubled theirs and Mastodon's is already 500 characters.
That and the main developer of Mastodon is working on media features to make it work better as an Instagram replacement as well.
The real secret sauce is that the Fediverse and AcitivtyPub means if you want a Facebook like experience you can use Diaspora and still friend / share / communicate with people on a Mastodon instance. Different programs can emerge as fews into this data graph but its still the same fundamental primitives (soon to be a web standard) data types.
Speaking of Riot, I'm hopeful the developers will look into integrating it better into said fediverse. It should probably at least support some means for Mastodon hosts to easily spin up a Matrix server at their Mastodon domain with no friction and the same account database. It will probably take Matrix protocol revisions to make that work, though.
Probably one of the important killer but missing features of Matrix will be the ability to integrate into websites the way Facebook messenger does, so you can have the social media site with the popup chat frames.
The only missing feature in my mind after all that is something akin to Facebook / reddit comment threads / disquis / discourse. So that non-social media sites can integrate social media comment sections into their media that either autogenerate threads, repost to a Mastodon instance / on Diaspora and let Fediverse commentors just comment right there on the page.
I've a carefully curated list of people I follow and Twitter is 100% useless with the algorithmic sort. Get a client that has a strictly chronological timeline. For example, I use Flamingo for Android and it's great.
So far riot/matrix seems like it pretty much strictly dominates slack and discord, at least for my purposes.
I know some facebook friends who have started using slack as a way to keep in touch with strong-tie friends, as a way to rely less on facebook and facebook messenger. So riot/matrix could reduce my reliance on facebook in some ways, too.
I haven't tried it out yet, but it seems mastodon is the most likely twitter replacement. Twitter has sucked for me starting about six months ago when things got algorithmic, it totally screwed up my curation.
As for the rest of facebook... I don't know. I was reading this thing about addiction, and how the bitch of addiction is that not only is it the thing you're addicted to, but it is also the thing that makes it harder to kick the addiction. (In the case of a substance, it hacks your brain to make it more difficult to resist impulse/urge.) Seems like something of a parallel, in how using facebook makes it harder to leave. The only path out in those cases is to take slow incremental steps that make it easier to resist over time. So, carving out the pieces of facebook that are important to you. For me that might be a public blog to share my thoughts, an email list, actually gathering email addresses and phone numbers for people on facebook I might want to stay in touch with, etc. Although I still want to check out Disapora and Scuttlebutt... and I don't know if ActivityPub is relevant here.