I'm not sure I understand the way your post relates to mine. Are you genuinely asking why BBC needs Twitter and Instagram? The last sentence makes it sound like

I'm not sure I understand the way your post relates to mine. Are you genuinely asking why BBC needs Twitter and Instagram? The last sentence makes it sound like you have your own hypothesis but you're withholding it. In any case, I disclaim any notion that "need" is a relevant verb here. They want to be on these platforms, and their readers want to be on these platforms, and there's no doubt some interrelation between their desire and their readers' desire. I don't think any organisation is ever fully rational, even an organisation the size of the BBC. On the other hand, I think the journalists themselves provide a lot of pressure for themselves to be on these platforms. Ultimately they're where it seems to make sense to be. It would harder to not be where the eyeballs are, than it is to be there. It's like the old saying - no one was ever fired for choosing IBM.