On facebook, only some pages allow this feature. If a post allows this, you will be able to check by clicking under your friend's comment. If it shows your picture and the "add a comment button", you click on this box and type your comment.
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unless you are on a discussion wall / a mutual friend's wall, you cannot like someones comment or status
The comment is posted immediately.
If you click on their wall a small box should be at the top where you can type and then you click send and it appears on there wall
You don't need a html code for comments. You just post a comment, or post a comment and wait to see if the uploader approves it.
It is not possible to post a comment on YouTube without an account. Posting comments on Youtube is a prvilege you get with having a YouTube account as this also attempts to deter potential mass spammers and cyberbullies.