According to Vicente's Rules of Grammar that depends on whether the smiley face applies to the whole post, a paragraph or a sentence. If it is enclosed within a period, exclamation point or question mark it applies only to that sentence. If you put it at the end the last period of a phrase or paragraph it applies to that paragraph. If it is placed at the end of an entire post with one or more spaces separating it from the last sentence it applies to the entire post.
If it is used in place of a punctuation mark it is considered grammatically incorrect.