@Chanders i've also been thinking about this, and a few inter-related complications:
1. workplace organising? (e.g. strikes, boycotts, walkouts)
2. labour implications of employer-provided official social media (not least metrification - though with caveats from @Angelec's work on how this plays out)
3. more-than-professional sociality (is one mainly using social media in official capacity?)
4. moving jobs/roles (without losing posting history)
given these things, wonder if there's (also) a place for professional/disciplinary associations (including, e.g. #AoIR), for more worker/employee-based instances (community-of-practice-owned, rather than employer-owned)? which could even be supported through things like sliding scale member dues?
in case of universities, i suspect IT policies may make this very slow to happen for many institutions. in meantime, perhaps we can look to support independent instances through things like research budgets to boost capacity?