Social media is a con trick. In fact it's at least three cons; Conversation, Content and Consistency.
Conversation - You need to get involved. It's no use simply posting messages saying how wonderful your company is, or what an interesting blog you have just written. You need to respond to questions, add to debates, and offer a point of view on issues. That's what engages people. Don't be scared of getting into a debate and then leaving it again, since it may run for days or weeks. Simply add to the debate while you are there.
Content - You need to offer something useful and of interest. This may seem contrary to what I said above (I'm even debating with myself here), but if you post an interesting and valuable article, other people will publicise it for you. Your comments on other people's blogs may also offer useful content, so don't simply say "I agree" or "This is rubbish".
Consistency - you need to make regular appearances. That does not mean every day, and certainly not every hour. However, if you only appear once or twice a month, post a ton of material and then disappear again, you won't attract many friends. Small, regular postings seem to be much more effective than rare long ones. Little and often - that's the way.
Of course, there is also Connecting, Confidence, Consideration, Congratulating, etc.