Well, to be clear, with that definition everything we do and say is social media. And in a way it is. Now, that removes freedom of speech because everything, no matter what and in what circumstances, can be used against you. Be it in jest, being out after a few drinks, or enjoying a nice night with company.
I guess in the UK people are a lot more relaxed about stuff like that than elsewhere. In germany recording calls without expressive consent of all participants is illegal and can land you behind bars (and in that case out of a job as pilot, even if it is only a suspended sentence you would not be allowed to go airside. Whatsapp messages are actually protected by the same kind of law, however, larger groups (certainly not 3) would be seen as either public or limited public statements. Even normal ATC radio is protected and listening to it without any need is actually illegal, broadcasting ATC radio via the internet will be shutdown and possibly prosecuted by a government agency.
That said, someone mentioning to friends that he is suicidal should be pointed very directly to the relevant professional help, and in case of pilots that is of course the mandatory peer to peer program that protect the job while things get sorted out without the company knowing what exactly is the issue.