You might be interested in the results of the 2016 GOV.UK assistive technology survey. [0]
iOS VoiceOver ranked as one of the 3 most popular screen readers, second to JAWS for Windows [1] (closed source, paid) and above NVDA for Windows [2] (closed source, free, donations encouraged).
ChromeVox [3] is AFAICT the only free and open source screen reader that came up in the survey, at 1%.
- [0] https://accessibility.blog.gov.uk/2016/11/01/results-of-the-...
- [1] https://www.nvaccess.org
- [2] http://www.freedomscientific.com/Products/Blindness/JAWS
- [3] http://www.chromevox.com