Since social networks are free to use, their primary danger is wasting of the time you invest in their use. Here are some details on that and other common pitfalls.
Time sucks.
Many people who are active in social media spend a lot of time online. Many of them are young and / or single with apparently had a few obligations. With plenty of free time for socializing, social media is truly a part of their lifestyle. This may mean that they are willing to invest more time into social networks than you can afford or want to afford, especially given the potential inconsistent ROI from social media marketing.
Conduct unbecoming.
Mixing your business or daily job and social lives can also lead to difficulties. Do you really want your contacts to automatically be alerted that you've posted your vacation photos online especially those pictures of you drunk in nowhere?
Overdisclosure.
The web's memory is long and possibly permanent. You need to think twice about everything you post online, especially if it potentially crosses the line between social and business or work, as mentioned previously. There are more stories every day about job applicants losing out on jobs because of risque or otherwise inappropriate material that company reps found on their social media network profile pages. And I warned you, Facebook party photos have been using by prosecutor to get longer jail sentences for drunk driving convictions.
Friend Overload.
This is variation of the time suck issue. This is where you connect with so many people online that you can't keep up with all updates. You're real relationships get lost in the noise. Often this lead people to log off entirely or quit and start over with the new account.
Don't Spam.
No one want spam unless it serve with bacon, cheese and bread. Spam usually refers to unwanted e-mail promotions, and all the new ways to communicate via social media networks offer you many new ways to annoy people. Yes, you an use social media as tools for promotional outreach, but don't be evil. Spam is still spam.