Facing Up To Facebook Scams
It's apparently called "Like farming". I'll paraphrase what the article says:
- Your friend "likes" a photo, for whatever reason (we'll get into the reasons in a moment)
- You see that they liked it and you decide to like it too.
- The photo goes viral and receives tens of thousands of "Likes".
- The person who posted the photo sells the group the photo was posted in to some shady product/brand.
- That product/brand now has a group with thousands of likes on one of their photos, and in return — because you "Liked" their photo — they can share their advertising with you on your timeline.
If you Google "Like farming" you 'll find other similar articles about it.
You can probably minimize the like-related ads by going into your Likes on Facebook and cleaning them up, "Unlike" anything that looks suspicious or that you don't remember liking.