Simply because it is more personal, people know that they are talking to a real person not a robot. And social media takes long to get results because you have to first build trust with your social friends.
Tip: Do not send any links to someone you just met/added, build a relationship first. Use facebook fan page to promote products not your profile, do not post any links in your profile send all your friends (whether from twitter, myspace or facebook) to your fan page and post links to any offer using your fan page.
Happy Social Media Marketing
