Google switched Google Translate over just last year, November 2016. Considering Facebook is developing all this internally and the scale they operate at and the presumable hardware disadvantages (does FB have any equivalent of Google's TPUs?), switching over in production so quickly is impressive.
45 languages * 44 other langues ~= 2000 translation directions. There are ~4,500 languages with more than 1,000 speakers ~4,500 * 4,499 ~= 20,254,500 language pairs so that's a very long way from every language.
I guess they really don't need to cover all of them though as most of the people speaking a language with low number of speakers would be able to converse in other mainstream languages as well. I can speak four languages but I rarely converse in two of them as speakers of the two languages are usually comfortable with a mainstream language as well.