Here's an easy way to look at it. When you look at an advertisement on TV, what is the advertisement trying to tell you? It's trying to tell you why you should do a specific thing.
For example, have a look at the following:
My Blog - It has all the latest and hottest innovations in technology, news, and cool stuff for you to check out! Special promotions!
Of course, this is a rather extreme example. There is obviously grey area in between. For example, what if someone posts a Youtube link and says, "It's funny." Is that advertising? No, it's not. It's definitely not if it's posted in the correct section here on Southperry. However, if one were to post in the general section, "Come check out this person's Youtube channel! It has really funny stuff guaranteed to make you laugh", then that would be advertising. Can you see the difference?
So advertising has a lot of grey areas. However, the following should probably be used as the base to critically judge if something is advertising or not:
(1) Is the link well-placed? Does it make sense within the context of the conversation?
(2) Does it use words which are commonly used in advertisements (hottest, best, coolest, chic, updates, etc.)?
(3) Does the poster attempt to use common methods of persuasion to entice a person to click on a link (pathos, ethos, logos)?
(4) Does the poster have a financial motive to the material involved?