Hey, folks! Anyone ever tried to run an online contest?

Yes. I notice that you said your contest is like a "treasure hunt" so this may be relavent for you.

I'm doing one for Alchemist Camp that's currently in the early stages. The site is a set of programming tutorials (screencasts for the Elixir language), and those screencasts include the actual building of the site I host them on.

In those early videos and in the site itself, I hid a secret easter egg. For months, nobody found it. I started making subtle clues on other pages that lead to the secret page which was the main starting point for the easter egg. I also hinted at the existence of it in my bi-monthly email. It was kind of frustrating that nobody found it.

But then a user did discover the secret! I announced in the next email that "a hero had arrived" and unlocked the first secret and gotten access to more content. Since then, more users have discovered the secret and user engagement metrics have increased. It's too soon to say whether or not this will pay off in a measurable ROI kind of way, but I'm cautiously optimistic. The main thing I've learned so far is that challenges have to be far easier than I'd first imagined.

I'm definitely going to continue with this experiment. It's easier to get results with the popular black-hat and extrinsically-focused types of gamification (e.g. artificial scarcity, random reward schedules, completion badges, etc). But, based on my research, exploration-based gamification tends to lead to more positive long-term results on a number of dimensions and that's what I want since I've got a long-term membership site instead of a one-off transactional relationship with my customers.

Yes! I've used Tiny Contest.

I like it because it connects to my regular mailing list, and runs the contests automatically.

I've run a "do this and win this" contest on social media.

All I can say is make sure the "win this" is at least two times greater than the amount of effort required for the "do this".

You can't expect people to create videos or travel somewhere (even a kilometer away) and sign up for something if the incentive is a $5 gift card. It's a tough thing to figure out but once you start the contest, you'll see if the incentive matches the effort based on the number of people who take part.

Also make sure that your "do this" is something your target audience would actually do. Where I live most people are shy to say anything on camera. They'd rather dance or do a dubsmash than actually say anything. Culture is something you need to keep in mind.

I did and I can tell you why it will fail

What kind of contest?