I think it probably reaches more people with Facebook. Public services like the BBC usually try to have a presence wherever the audience is!
https://en.wikipedia.org/wiki/BBC
"funded principally by an annual television licence fee[12] which is charged to all British households, companies, and organisations using any type of equipment to receive or record live television broadcasts and iPlayer catch-up.[13] The fee is set by the British Government, agreed by Parliament,[14] and used to fund the BBC's radio, TV, and online services covering the nations and regions of the UK."
I don't see that they publish the same content anywhere else. Even for Facebook they could have put only links to their own pages with the same content, had it existed. But it seems it doesn't.
>You own the intellectual property rights (things like copyright or trademarks) in any such content that you create and share on Facebook and the other Facebook Company Products you use. Nothing in these Terms takes away the rights you have to your own content. You are free to share your content with anyone else, wherever you want. However, to provide our services we need you to give us some legal permissions (known as a ‘license’) to use this content. This is solely for the purposes of providing and improving our Products and services as described in Section 1 above. [...]
>This license will end when your content is deleted from our systems
In this specific case I as a potential viewer of that content can't visit BBC's site to watch the same content, I have to watch it on Facebook, if I want to watch it at all. And I've searched for the specific video on BBC's site.
But BBC's mission isn't to make Facebook earning more money by exclusively providing to the Facebook BBC's content, helping Facebook earning more money from the internet ads tracking their viewers, and moreover, it's not why BBC is publicly funded, and why it collected its fees to produce the content: note again what I've already quoted: the "annual television licence fee which is charged to all British households, companies, and organisations" "is set by the British Government, agreed by Parliament,[14] and used to fund the BBC's radio, TV, and online services."
And it's not that BBC doesn't have its own web site.