Inappropriate Use Digital Media

This is not a huge security issue, but I have had acquaintances ask to be my friend on facebook.  I was unsure if I should accept, but ultimately have done so in most instances. However, they were often trying to sell me something or get me to join something.  It is disturbing that because they knew me in some capacity, they were trying to really contact me only to get their foot in the door for their own benefit.  There are many inappropriate uses, but to me this is one...as it comes across as a friend request but really they are not proposing friendship at all.  What might have been done differently is that person contacting me through email asking if I would be interested in a product, rather than doing it on social media.