How companies make use of Social Media matters. If you deal with a final consumer you should be active, but not just ploughing in without thought. Social media is a conversation not a broadcast media, and companies are tolerated on Social Media only if they understand how to participate. Business to business activity is also relevant but again must be carefully planned or more harm will be done than good.
I have done quite a bit of work helping business work out how to approach the extensive mix of Social Media. Not all companies see the relevance but all have reason to monitor the area and more than you imagine need to be involved. Below is what I have been putting on Twitter in the last few months in the form of a cloud:
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