It is hard to predict the number of Esperantists in the world. But there are at least 100 thousand English speaking Esperantistaĉojs in the world.

Source: https://www.duolingo.com/course/eo/en/Learn-Espera...

If this is the number of English speakers using Duolingo, then certainly there must be at least 2 million Esperanto speakers in the whole world. Keep in mind that Duolingo requires access to a computer and internet. In addition people have to know about it. There are probably tons of people who haven't heard about this app.

Let's just say that at least 2 million people in the world can at least say "Saluton".

From Estas mi Esperantisto by Julio Baghy

Mi parolas kun rapido:
"Bonan tagon! Ĝis revido!"
Ĝi sufiĉas por ekzisto;
estas mi esperantisto.

By that criterion, one needs to be able to say Bonan tagon and Ĝis revido as well.

Alkanadi:then certainly there must be at least 2 million Esperanto speakers in the whole world.

Why?
As for Duolingo, it is unclear how the learner numbers are calculated. Does everyone who clicked the pictures for 10 minutes once count as a learner permanently? I could not find that kind of info on their site.

Armand6:As for Duolingo, it is unclear how the learner numbers are calculated. Does everyone who clicked the pictures for 10 minutes once count as a learner permanently? I could not find that kind of info on their site.

I believe that is how these webistes work because they want to exaggerate their numbers

Learner numbers on these websites are anyone who has ever signed in to look at that language. If you counted that towards speakers of a language, then I "speak" German, Dutch, Portuguese, and Spanish. I assure you that I speak none of these languages.

I believe the current number of people who have finished the Duolingo lessson tree for Esperanto is around 300--I don't have a citation I just recall reading this from one of the developers at some point. Of course, I never really did any lessons on Lernu, so whether someone finished their online lessons or not isn't a perfect indicator of whether they do or don't speak the language.

Now like you said, remembering that these things require an internet connection and not everybody uses them etc. etc., we do have the number of users on Facebook who list "Esperanto" as one of their languages. That number is 310,000. The number of people who "like" the Esperanto language page is 32,736 (meanwhile 20,000 people "like" the Lernu page), and the number of people who are members in the main Facebook Esperanto group is, as of today, just over 18,000. The Duolingo Esperanto Learners group has 2,310 members at the moment (the group is about as new as the Duolingo course), and 26,733 people joined Lernu in 2014.

Roughly 18,000 people who have Facebook accounts have gone as far as to join the Esperanto facebook group, 32,736 have "liked" Esperanto and 20,000 have "liked" Lernu. I think it is safe to say that the lower end for our estimates of how large our language community is, at least in its online form, should be above 10,000.

The New Zealand 2013 census counted 108 Esperanto speakers. The 2011 Hungarian census counted 8,397 Esperanto speakers. The Russian 2010 census counted 992. The Lithuanian 2001 census counted 844. The average of the amount of sign ups at the last 10 UKs is 1551 (Lille this year had 2695 people signed up). In 2014, UEA had 5,027 individual members and 9,789 linked members. Now, these are all flesh and blood people either counted on an official census (still, I guess they could lie), or who paid money and/or traveled distances to be involved with Esperanto (probably not something you would do just to be funny). Now, definitely way less people have the time/money/willingness to attend a UK or pay a member fee for UEA, than have access to Facebook, Lernu, Duolingo, and other social networks. So while these numbers are some of the most verifiable, I also think they are much lower than the actual size of our language community. The ones associated with UKs and UEA especially involve more money, old age, and privilege than does having a Facebook account. But on the flip side of that, one person can have 5 Facebook accounts and list Esperanto as a joke on all of them.

I think looking at the actual posts per day and different IPs and such in some large Esperanto forums will start to give us a closer picture of who is really using the language, along with contacting people who attend clubs and events in the "real world" and gathering information about their experiences.