Depending on the website, a pretty wide array of personal information such as employer, age, location, address, relationship status, information relating to your device, and more. This data can be sold or shared to other companies or to individuals. If you've posted photos or videos of yourself publicly, this can all be used to train an AI to emulate you.

I cannot stress this enough, but this is a good reason to never put overly personal information online. At most I'll only share my name because there are quite a few other people who have it, but never my age, photos, location or anything that could narrow me down to one person. Even without corporations selling your info, there can be malicious individuals who can exploit it through various means such as identity theft, doxing, etcetera.