There is little doubt that, foremost among constructed languages though it is, Esperanto has not — particularly in recent times — captured a sufficient amount of general attention to become the functioning worldwide auxiliary its proponents wish.
1995, C. K. Ogden, Psyche: An Annual General and Linguistic Psychology 1920-1952, C. K. Ogden, ISBN9780415127790, page 13:
Study courses of Esperanto and Ido have been broadcast. In the possibility of radio uses of a constructed language — and such experiments are proving successful — vast sums of money and untold social forces may be involved.
The first practical constructed language was the south-west German Pastor Schleyer's Volapük from 1879; its complicated grammar and irregular vocabulary made learning difficult, however. The most successful has been Esperanto, devised by the Warsaw ophthalmologist Ludwig Zamenhof in 1887, that today can count some one million speakers.
For him Esperanto is principally a language of poetry, which he uses to express his feelings and thoughts. He also writes poetry in Russian, and some poems of his exist in both Esperanto and Russian versions.