That's tough. I wouldn't say it's views for popularity, especially because that number can easily be affected by a time frame (100 views in a day could be good, 100 views in a week could be not so good). That being said, the top 120 popular for all time on this site are all in the millions of views.
In my opinion, so long as you have that 1 reader each update, then the story is popular. But for the traditional view, I'd probably say at least 1000 viewers subscribed (or whatever it's called on here).