TourRadar is a great case of how to carry out a giveaway/lottery of travel (or, actually, any!) product or service to achieve a good balance of involvement and efficiency.
The usual problem with giveaways is that if you have a low entrance barrier to the lottery (e.g., a like or a comment), the participants would be more eager to join the contest, but the spread of information and your reach to new audience would be limited. On a contrary, if you require a "Share" or a personal post with a mention of your contest, the reader may be more reluctant to participate.
Tour Radar solves this issue by offering differentiated options for participating in a giveaway. The initial "entrance barrier" is minimal – you just need to enter your email and name. Then the website offers you to increase your chances of winning by sharing the link to the contest on social networks and by doing a "treasure hunt" – clicking through the company's website and finding some information on it. It's also a psychogical trick – people are much more likely to share content to "increase their chances" when they are already inside the loop.
And cherry on the top – the company would further re-target the participants by sending them marketing emails once in a while. Overall, great promotion case!