According to MarketingSherpa (open access through March 2nd) there are about 75M RSS users in the US and the UK. Of those, nearly 50M have no idea that they are actually subscribing to RSS feeds – and most would never click on the little orange RSS button.
So who are those people – they are the users of MyYahoo! and MyMSN. And according to Nielsen, even though they may be unaware of use – they are influencers nonetheless.
The article also mentions a best practice related to this. When Travelocity announced the availability of an RSS feed to their users that have Yahoo! or MSN accounts, 2/3 signed up for the feed.