traffic-exchanges-explained

Whish is the more prductive source of traffic today - traffic exchanges or social media sites such as twitter or facebook. And in terms of traffic which is the best source for targeted traffic to your site. Personally I fail to see the great importance currently attacked to a site such as twitter in spite of its current popularity. It seems to reflect that we are basically lazy and that 140 characters is about as much as we can say or digest in one go.
However sites such as delicio.us and diggit do bring real results in terms of interested and targeted users to the site.
As I stated in an earlier posting I fail to understand how traffic exchanges can advertise that they bring "targeted" traffic to your site. I understand that the traffic can come pretty quickly once joining and surfing a traffic exchanges and I understand that the TE can be useful for getting a list of email addresses but again how is traffic targeted?