When you run a social media competition when people submit pictures or videos and one person wins a free prize, DO NOT use your own service as a free price. You want to use something that people already want. Usually these are things like Apple products or cameras. If your service is so valuable that it will incentivize people to submit for a competition, they like your service anyway and for you this “marketing” campaign is redundant – only people who already think you have the best service ever would participate.
Also, don’t give out the prize quietly. Make it the biggest announcement in the world. It’s special.