Here's why: 1) The people I have as friends on facebook have varied interests, and usually it's mostly smalltalk, because even if I ask people about their interests, they would rather not explain them to me on facebook, or they do a very bad job of providing insight into said topic. There's no real value to anything being said, and yet this happens almost everyday.
2) My closer friends have interests more inclined towards mind, and often, our discussions, lead to new ways of thinking about said topics (usually 'intellectual' [humour me here] stuff), sadly this happens maybe once a month or so.