There are many reasons why you should be monitoring all your social media outlets. http://us.cision.com/assets/Cision-10-Reasons-Monitor-Social-Media.pdf lists 10 reasons. The top 5 are:
1) Reach ready prospects- Social media gives instant insight into the habits of your best prospects.
2) Discover the influencers- Social media allows you to discover the journalists and bloggers who have the most influence on your topic.
3) Spot the trends- Social media is like seeing the future. By monitoring it you will be able to spot the trends that will be making big news tomorrow.
4) Monitor the competition- Using social media you can see what good things and bad things people are saying about your competition.
5) Manage any crisis- Social media allows you to respond to a crisis immediately.
http://socialmediatoday.com/pamdyer/1458746/50-top-tools-social-media-monitoring-analytics-and-management-2013 has a list of 50 tools for monitoring social media. Some are free, but most of them cost a little bit of money.