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If you click a circledot on a post, it means "share", or "I want my friends / followers to see this"; also adds the post to the "posts and shares" section of your homepage (which makes it easier for you to find it in the future).
If you click a circledot on a comment, it kinda means "upvote". The fact that "upvotes" are limited to a "full circle" is a feature, not a bug. Keeps it from becoming a thing to focus on.