There's about three main kinds of social applications we use - messaging apps (iMessage, WhatsApp, regular old texting, etc.), social/work media (Twitter, Slack, etc.), and video conferencing (ie Zoom, Skype). (Note that sometimes messaging apps can also include calling and one-on-one or small-scale video chat).
The problem with these is they are closed-source, often relying on fremium, data-harvesting and/or ad-based services. Now it would suck if these were our only choices - but they aren't. Long-story short, for messaging there is Signal, for social/work media there are many choices, but Matrix is a viable option. Finally, for video conferencing there is Jitsi.
First up is Signal. Signal is very easy to get into, and very useful. It is the most secure messaging application, and has the most secure (although not perfect) calling and video chat.
Next is Matrix, which isn't quite an application itself, but a protocol. Now I suggest Matrix for a few reasons. First, the bad news. The flagship server/application - Element (sort of like how WhatsApp was an application based on the XMPP protocol) - is centralized. Now I'm not too concerned with the Matrix non-profit, the problem is centralized servers are a bit of a security liability in my view. But this isn't any more of an issue than with any proprietary application.
HOWEVER, Matrix has a unique and important feature - one of its central goals is to act as glue between different social media 'silos'. As it is, we are often 'siloed' into certain social medias - when you're on Twitter you can only engage with Twitter, when you're in Slack, you can only engage with Slack, and so on. But what if there was an application which bridged these together? And what if this 'glue' was itself a FOSS application? That's Element, or any Matrix-based application.
Basically, Matrix isn't just a FOSS application - it's a bridge service which connects people on many different platforms. This isn't to advertise those other platforms, but to encourage you that going on a platform like Element isn't a dead end.
Finally is Jitsi. Jitsi can be downloaded as a web app, or can be used as a web application. It's a great video conferencing service, as functional as Zoom, but FOSS.