There is a 100/10/1 “rule of thumb” with social services. 1% will create content, 10% will engage with it, and 100% will consume it. If only 10% of your users need to log in because 90% just want to consume, then you’ll end up with the vast majority of your users in the logged out camp. Don’t ignore them, build services for them, and you can slowly but surely lead them to more engagement and potentially some day into the logged in camp.

Source