It is no secret that we are huge fans of RSS around here. (Which, I know, the irony, this is a newsletter tool, etc.).
You can set up your newsletter to send directly from RSS, you can automatically publish an RSS feed from your newsletter... but the one downside of RSS is that it is hard to know how many people are subscribed to your feed.
Recently, I learned from Darek Kay's blog that many RSS readers actually report the number of subscribers in the headers of the requests they make! So, I added a new feature to the analytics page that will show you how many subscribers you have to your RSS feed.
This is free for all users (including free users) and you can see it on your analytics page. There are only support for four feed readers right now (Feedly, Inoreader, NewsBlur, and Feedbin), since as far as I can tell those are the only ones that report the number of subscribers in the headers. If you use a different RSS reader, let me know and I can add support for it!