I check Facebook 2 or 3 times a week to see what family and friends have been doing. With Twitter, I follow people into the same technologies that I am interested in, and I get a lot of interesting links.
re: gmail: I have never bought anything from a sponsored search link, but I do occasionally buy things from advertisement links on my email - also, sometimes I get an email from a customer asking me to help them with a problem and a useful link is on the right side of the customer email.
Sure, privacy issues are huge, but I think that people need to make a personal calculation of benefits vs. potential problems.