On my website I am using the 'Share' button to post a comment, document, or to have a poll. I'm confused either to use 'Share' or 'Post'.
Nowadays 'Share' is understood as sharing towards other applications or social networks. I would find it confusing to see 'Share' instead of 'Post'.
Example below while i am answering this question, the label of the button to validate my answer is 'Post Your Answer', which makes more sense to me than 'Share Your Answer'.
Share is a far more human term with a clear indication that there is some relationship between you and some other people. It implies some sort of connection to the people that you are sharing with.
Post is a publishing term with very little concept of a connection with someone. It is clinical and cold.
So if your intention is to share with people, use 'share'. If it is to share with the world (i.e. publish it), then use 'post'.
I think "Share" would be better suited to when you are sharing already existing content with others, and "Post" would be better for making new content that is available for viewing by others.
Think of a bulletin board and the similarities between it and a social networking site for example. You would post the new content onto the board, but if there was something already on the board that you wanted to show to others, you would share it