Hi there! I'm mostly picked for mega giveaways with 3000 winners (both skin1004 ampoules) and local, country-limited giveaways, which were Ruruberry and By Ecom sets for Malaysian residents.
There were some controversies regarding the factors that affect our chances to win but most people agree that your demographic information and location are the main reasons for winning, plus an element of randomness. For example, if the brand is targeting to enter the market or expand in your country, you're more likely to get selected.
I can't say anything about how common are giveaways for new users since I joined more than a year ago π But I can assure you that even if you're a new user, you still have a chance. I once saw a discussion about how a user won even though she didn't have any comments, discussions, reviews, basically no activity on the app π Hope that helps π₯°