I've observed this kind of conversation among young people many times. In fact, I often wonder why high school kids NEED to have a phone stuck to their head 24/7, because the conversations apparently never really get beyond "w'sup?". I'm so glad cell phones hadn't been invented yet when I was that age, because I would have been at a total loss. Maybe I'm officially an old fogey.
Now, I'm no conversation wizard, obviously, but I believe that when someone comes up and begins a conversation that way, it's a greeting, and you should not feel any pressure to keep it going. A little silence never killed anyone, and it's okay to stand around watching the cars go by, or just give a friendly nod of the head. I honestly don't know if people see this as a fault or not, but I generally won't say something unless I have something to say. If the other person has something to say, they will, and then you can respond to it with a lot of talk or with a "yep" or whatever feels right at the time.
Then again, I'm completely aware that other people have some kind of inside knowledge about this stuff that I don't have, so maybe I'm way off base there. Perhaps the whole reason I'm socially isolated is because of the fact that I don't try hard enough to keep a conversation going. But, in general, if people are uncomfortable with my lack of constant, immediate responses to non-answerable greetings, then they aren't letting me know about their discomfort. Or perhaps I'm just not getting the hint.
This is all from someone who isn't sure whether she's an aspie or not.