On a personal level, I think that social media is a huge negative. It can promote unsocial behavior. With new features and games coming out that accompany social networking sites, there is little to no reason to go outside to chat and do things with friends when they can be done online. Furthermore, it can distract the user from doing something productive, like doing assignments. Also, employers tend to use social network profiles of job applicants to judge how he/she would act in a professional setting.
On a societal level, social media has only one impact, and it is a very amazing one. Social media makes spreading very vital information, such as information about a crime, simple. I recall seeing a news station covering the numerous amount of twitter posts about some crime in NYC.
Do I use social media? Yes, I have a Facebook account and I check it daily, but no more than twice a day. Any more than that feels like a waste of time to me. It's also important to refrain from posting anything immature on networking sites. Social media is a tool to promote yourself and connect with your friends and nothing more.