You receive an email with a question asking for advice or a suggestion or an opinion. To give a full answer you would have to take some time to think. You are a little busy and you would rather not give it too much thought but there is a second consideration that leads you to give the quick and dirty answer right away. The longer you wait the longer they will know you thought about it and the more credence they will give your answer. Not to mention that more of your reputation will be at stake if you are assumed to have thought carefully.
Still, some issues are important enough to give thought to. But how much? The same tradeoff is there, but now the characteristics of the correspondent matter. Every additional second you spend thinking allows you to make a slightly more thoughtful answer but also increases what he expects of you. If he is very sharp, he will be read your reply and possibly see deeper into the question than you did making you look bad. The gap only gets bigger the longer you wait. If he is less sharp, every second tilts the balance in your favor.
All of this is predicated on him knowing just how much time you spent on the question. You want to manipulate this by establishing a reputation for rapid-fire responses. Then if you wait a day but still give a lousy answer, he will put it down to you just having been busy for day before giving your usual top-of-your-head reply. Indeed you want everyone to think you are busier than you are.
Then along comes instant messaging, facebook, etc speeding up communications. You are expected to have seen the message sooner so its harder to pretend you were unavoidably delayed. On the plus side though now you can more easily commit to being busy. Just friend everyone. Your feed is so cluttered up with babble that these really important questions credibly get lost in the shuffle. He can directly see how overloaded you are.
So the value of your marginal friend is equal to the incremental publicly observed distraction she creates.