After all, as a developer, it's my site - I choose the URLs (including the query strings) which are valid and acceptable to me.

I wonder how many sites other than Twitter are rejecting requests with unknown query parameters?

I would consider it to be pretty bad practice to treat query params this way, extra query params should be ignored. However, web standards are descriptive, not prescriptive. If enough sites have strict requirements about the number of query params, then it doesn't really matter what good practice is, and Facebook should accommodate this by moving back to cookies or at least make it opt-in or something.