The San Fransisco font is only on the Apple Watch at this time. iOS 7 and 8 use Helvetica Neue as the system font. It's main issue is/was that it look terrible on non-retina displays due to it's thinness. There is a setting in Accessibility that helps to some degree, but replacing it with a bolder version of the font.
I have no idea what Safari's reading mode is using, but since San Francisco is not installed on iOS 8 devices, it's not that.
The San Fransisco font will be coming (so we hear) to iOS 9 and OS X El Capitan. Most font buffs like it better than Helvitica Neue. Me, I'm not that picky. I'm not a font expert. I just read a few blogs where font lovers go on, and on, and on, on about fonts; and drool over the upcoming San Fransisco (at least as a system font).