On Duolingo there are currently 501k learners on the English Esperanto course (no number is given for the brand new Spanish one). Obviously they will not all speak it fluently, but I'd say that this would have boosted older estimates by a few 100k.
Jiri Hana in his 1998 Master thesis quotes Funk and Wagnall's The World Almanac, part of "Microsoft Bookshelf 1994" as giving an estimated range of between 1 and 10 million speakers, with about 1000 native speakers.
The 1000/10,000/100,000/... estimates do look suspiciously regular to me (and are 20 years old), so I don't trust them. I have also come across the 2m estimate before; this seems reasonable, soand with a projected recent increase due to more learning opportunities my own guess would be about 2.5 million now (2016)