How to fix “swipe up” and “swipe down” gestures in Reeder after installing Lion?
This problem is a known issue and has been fixed in recent versions.     

However, it is not specific to Reeder and seems to also affect Twitter and other apps.  

To work around it, hold ⌥ Option and swipe using the same count of fingers that you have set for Mission Control in Trackpad preference pane: