With social media you can theoretically see how many people saw your post. (I say "theoretically" because you don't know if they are giving you accurate numbers or not.) There is even the eye candy of the total number of likes and shares.
And while it is somewhat better data than you get from, say, a magazine ad, it is still just eye candy. There are so many people out there who, when they see content that they like, don't do anything. They don't click like, they don't comment, and they don't share. Why? Because they don't have to. They grew up in a world where, after you read an article you liked, you put the magazine down and that was the end of it. Maybe you mentioned it to a friend, but you did not document this communication in any way.
No matter what the platform is, the total number of likes you get for a post is not an accurate measurement of the number of people who read it and liked it. And it is definitely not an accurate measurement of the number of people who read your post and thought about it.
Social media, blogs, and such are incredibly powerful and valuable ways of communicating with your audience, fans, or customers. However, there is no good way to measure any of it yet.