First of all Milestones are unique in each Person's eyes. I can set a Milestone for 100 while my Friend can set a Milestone for 10, both are considered Milestones but only to the Viewers Perspective.
There are now over 35 million unique Ethereum addresses on the Blockchain which is huge and in my Eyes can be considered as a Milestone in itself. Interestingly the number of Unique Addresses of Ethereum is more than the Number of Unique Addresses of Bitcoin. It goes to show the overall Popularity of Ethereum which is the Second Largest Cryptocurrency in Market Cap.
