A vast majority of what you read in facebook statuses, online articles, quora answers, have nothing new to say to you. You see cliches everywhere - things that have been beaten down to death in your head, and evoke little human emotion or 'aha' in you. Makes you generally undemonstrative.
In most debates, you are intimate with the cogent arguments of both sides, so you're rarely find yourself taking a stand with one side or the other, because you see how both have some great points and also how both are willfully or ignorantly blind to the great points of the other side.
Reading a lot is bad for the elasticity of your brain, and hampers genesis of creative ideas, especially if you read a lot on the same topic, if you specialize. If you read eclectically, this vice is limited.
Reading takes away from time you could spend outdoors, being active in the real world. And it takes away a lot - it is one of the most time consuming activities. The only respite here is if you're out in the real world a lot, you know that the real world is a little over-rated.
Reading ingrains behavioral biases in subtle ways that you have to always consciously guard yourself against, which takes effort.