Facebook seems to use any language they want to at any time. I don't consider it a major endorsement that it appears in production, it just lets me know its on their radar.
If Google picked up D I would be much more interested as to why.
Having D used in production in a big company like Facebook is a big deal. No, it doesn't mean that D has taken over the programming world or anything like that, but it shows that the language is maturing and that real companies are now willing to use it in the real world - and it's a pretty major company at that.
So, this is just one step forward, but it's a big one and exciting to the folks who are fans of D - especially those who have put a lot of time and effort into it.
Seeing D in production is a good thing. Google picking up a language doesn't prove anything. Seeing a language in production in a high volume setting is impressive.
If Google picked up D I would be much more interested as to why.