Hi Andrew, Phoebe -- here's what the Communication department here is comfortable with:
Wikipedia and the other projects operated by the Wikimedia Foundation receive hundreds of millions of unique visitors per month
The numbers of unique devices for enwiki are far greater than 500mm. But of course, people use more than one device and/or shared computers. There are datasets out there about average number of devices per person so we could potentially use this as a scaling factor to get a higher level of confidence but IMO the mapping from device to actual human is always going to be dicey.
For comparison, I worked at Yahoo for a long time and generally understand their tech stack -- in their 2014 annual report, Yahoo speaks to "more than 1 billion MAUs".[1] From my experience, I really don't know how they could measure this with any certainty without estimation or other statistical techniques because they have the same measurement issues that we do. Only Facebook or other sites where personalization is necessary for the site to work can report on reach without some sort of qualification.
-Toby