Most banks, and all the big ones I know about, have some level of on-line social media presence and dedicated social media managers (and wildly varying levels of competence with social media). As they say, 'that's where the customers are'.

The financial I work for has official channels on most platforms and dedicated teams who do basically advertising and customer support type stuff via social media channels. Complain about something on @OfficalBankChannel, and someone can see it and hopefully respond positively. This isn't just some random employees slacking off on Instagram or twitter or whatever; it's their job.

So if the bank perceives someone is threatening people over a social media channel, it can be treated the same as someone threatening over the phone to a call center or in a branch location. And there can be consequences.