I think it is. The Open University’s Moodle is often cited as “one of the largest”, but the OU has 250,000 students, as against our over-2,000,000 registered members.
The US military has a system that “serves 1.2 million people” which is impressive, but still smaller than Yacapaca.
The only candidate I can find for a larger system is Indira Gandhi National Open University with 3.5 million students but I cannot find details anywhere whether they have an integrated elearning system or not.
Whether we are actually the largest or not, with 80,562 teachers and 2,010,440 students registered, I think we have done quite well for a free service with absolutely no marketing or promotion. The growth has all happened because teachers have told each other about us.
If you know of a larger system, let me know in the comments.