How 'bout "every time you vote on a link provided by someone you follow, that 'follow' is extended a set increment of time." Set "follow" for 2 months. If you share content that you follow, the clock gets reset as if you'd just chosen to follow that person. If you don't share content the clock counts down. No shares in two months? You no longer follow that person. 1 share after 59 days? You still follow that person for another 60 days.
'K, set it to "number of posts" rather than "time." If alpha0 shares 20 links and you vote on none of them, you no longer follow alpha0. If alpha0 shares 19 links and you vote on the 19th, you'll follow alpha0 - no matter what - until his 39th post. Certainly incentivizes you to only share quality content, rather than shotgunning every single Reddit link you find.