The main issue may be ... Internet access in Mongolia: according to Wikipedia (emphasis added),
The Internet, established in 1995 in Mongolia, has yet to make a significant impact, with only 16% of the population having access to it as of 2012.
Melinda Soós, who was teaching English in Mongolia in 2010, wrote in The Guardian that the biggest challenge there was (emphasis added):
Access to English language resources: I have a whiteboard and imagination. Even if the availability of English resources, including teachers and the internet, improved, could professionals who missed out on learning English in school afford access?
So, another factor is how many Mongolians know English. According to Wikipedia, Russian is (or was?) the most important foreign language. However, according to an article on the Mongolia Focus blog (May 2016), English may have overtaken Russian as the most important/popular foreign language in Mongolia. However, the informal survey that provided the data for this statement was run on Facebook and Twitter, which may have a higher percentage of English-speaking Mongolians than the population at large.
The rising importance of English in Mongolia is the consequence of a politic decision just over a decade ago. See e.g. The new second language of Mongolia is... English (book of joe, February 2005) and English in Mongolia (LanguageHat, February 2005).
What I have written so far only shows that English is on the rise in Mongolia and that Internet access cannot be taken for granted. So where on the Internet do you find native speakers of Mongolian who know English?
italki currently has only one tutor for Mongolian, compared to 8 for Slovak, which has roughly the same number of native speakers. (According to Wikipedia, both Mongolian and Slovak have 5.2 million native speakers, but the census data are not from the same years.)
I don't see Mongolian on Busuu, LingQ.
I'll update my answer when I find more sites.