Whether it's a free vacation, a $1,000 gift card, or an iPhone, don't get your hopes up if an ad tells you it's your lucky day. Chances are, it's not.
Clicking ads like that to "collect your prize" is risky. Though it may look like a legitimate company is behind the ads, it's actually third parties or even scammers trying to collect your personal data. Just visiting a third-party website could end up installing malware on your phone.
Next time you get one of these ads, don't like it, don't open it, and don't share it. It's safest to close out of that browser window entirely, instead of just hitting the close button on the ad, to avoid accidentally clicking it.
If you seem to be hit with these ads a lot, you can try to reduce them by clearing your browser history.
Here are a few links with step-by-step instructions for reducing these ads on your smartphone.
How to stop pop-up ads on iPhones.
How to stop pop-up ads on Androids.
Another option is to do a full factory reset on your phone. Just make sure you back up anything important to an external hard drive or cloud-based storage before doing so.