how do people end up getting hacked on facebook?

[Mi]

Those are not ads. Those are actually "Click-Jacking". You may have seen some posts which are very interesting or usually containing adult content for getting attention (shared from your friends). These posts are not actually shared by them. When one of your friends click on that post, it contains a code which make their browser to post same content on their own facebook wall without their knowledge.
In this way their account got compromised.
Don't ever click on these types of posts!