What are.. these things?

Well, user-generated ads are a way for you to, without any money, promote your website/service. Not miiverse clones though.

Where do these ads shows up?

These ads show up in the activity feed, user pages, the home page, and more.

How can I display my user-generated ad on the site?

Contact sosig/smf9/term/Jokko, up to you and up to our timezones I guess.