There are jurisdictions (the EU, the UK, Singapore, Japan) with copyright exceptions specifically for text and data mining for AI purposes.
https://www.twobirds.com/en/insights/2021/singapore/coming-u...
https://www.twobirds.com/en/insights/2021/singapore/coming-u...
For the case of the EU, based on the way the GDPR and related digital laws are drafted very much in a "spirit of the law" and with individual rights and agency trumping corporate interests, it seems fairly likely it would not just allow coopting personal social media content over the explicit wishes of the creators, regardless of whether any access was deemed legal. For the moment, I think the UK digital laws are still basically just copies of the EU ones as well (with some search and replace) but I guess they'll drift apart over time if there's no de-brexit.
It's also worth noting, especially given how it's written about in the linked post, that these are generally assuming that these uses do not destroy the market for the original works, and they come from the reality before all the art generation. The specific implementations everyone is talking about, especially in the art space, have now pretty definitively proven that we're in a new reality now.