Most of these pros and cons are from my students' point of view. They just debated this topic last week.
--The sites have certainly made it more convenient to get in touch with people with whom we have lost contact.
--My students found several studies which demonstrated that social networking sites have helped business, especially small businesses.
--They can be helpful in networking for employment purposes.
Cons: (By the way, my students, almost all of whom use social networking sites, were surprised at how many legitimate cons they found in their research.)
--Predators abound on the sites (sexual, identity-theft, etc.). One of my students even set up a free e-mail account which she linked to Facebook, created a completely fictional teenage male, and was shocked at how much personal information others were willing to share with her. Moreover, games such as Farmbill can gather not only the player's personal information but also that of the players' friends even if someone has privacy settings on his or her account. I understand that Facebook is working to change this.
--While sites have helped some jobseekers network; they have often hurt jobseekers' opportunities. I cannot remember the exact percentage, but one survey found that the majority of employers search the Internet for information about potential employees and weed them out based on inappropriate photos, obnoxious comments, and the like.
--Once someone deletes information from Facebook, it does not disappear. In fact, my students investigated this when they were told by a career day speaker that this is true (he was warning them about ever posting questionable photos on line even if one's Facebook account is set to private). They found that if they Googled photos that they had deleted from their accounts, those photos were still available online.
--Of course, the decrease in face-to-face communication is often listed as the most significant con, and it certainly is one; but I have also encountered several situations where spouses have initiated contact with others through social networking sites because they provide privacy and opportunity that would normally be unavailable to them. Two of my high school acquaintances started "friendships" with old friends on Facebook and eventually left their spouses for their new "friends." Similarly, I cannot count how many of my high school students who have said, "My mom's addicted to Facebook." They then go on to explain how their moms talk to their high school guy friends and constantly check their Facebook pages throughout the day.
--The military is warning its members to be very careful of what they post on the sites because they have found evidence of would-be terrorists trolling the sites for personal information about military members.