Twitter or social media is not for massive casino traffic. Casinos post promo codes so their users be more active on their social media. That's it. Some so many players did make any deposits in the last few weeks. So casinos give some promotions to attract them. About new users, I don't think casinos get so many new users from their social media. Social media is just for activity purposes.
Casinos have other methods to get new users. They collaborate with affiliate marketing companies and offer them high commission rates. There are some ad networks as well to get new users. Sometimes you can spend money on social media boosting as well. Which will reach out to your target audience. But, they don't give you a guarantee that those users will sign up.