How do you get a medal for a club?

I recently got a 'Deticated Fan' medal for the 'Barbie in the 12 Dancing Princesses' club. Before, I thought the same as you. But after receiving the medal, I think it's actually 2 things.
1. Contributions, like you said.
2. How long you've been in that club.

Hope this is right and helps.

The exact formula for gaining a medal isn't known (they don't tell us or otherwise people would be cheating to get a medal). But it's known that you need to contribute to a club. Contributions which get a lot of fans and favourites, or even just get commented on/voted on for polls/answered a lot for quiz questions/looked at a lot, help. The system is a bit random. People can get medals in clubs where they've only added a handful of items, but no medal in clubs where they're a lot more active.