Reading, measuring and reacting to online buzz for brands is an extremely invaluable tool. The fact that so many people are talking about your brand and discussing your brand without your knowledge can scare some people but only because they aren't a part of the conversation. If you're involved in the conversation you welcome any feedback, even negative feedback, because at least you then have the chance to respond.
It's not possible to be involved in them all, but there are a number of tools and services that allow you to track them. I can't make any judgement in this post as to who you should use because each of them has some strengths and weaknesses, but overall it depends on the amount and type of information you are looking for as well as the time you are willing to invest in reading and reacting to this information.
If you want to peek at some of the companies, check out this list here. All of them are really strong:
Nielson Buzz Metrics
SEER by VML
If you've used any of them and you have some feedback, post a comment and let me know. I would love to hear what you think of them.