Open Source Intelligence (OSINT) is the collection of data that is publically accessible, typically from the Internet or "surface-web." This data can be aggregated to gather a better understanding of a company, an operation, or of a person's daily life. OSINT is typically used to learn more about company or government operations, but the same tatics can apply to individuals as well.
Searching for someone's name on a site like https://mylife.com or https://fastpeoplesearch.com can tell you information like where they live, have lived, email address, and more. So, for example, someone posting about going on vaction could be used to ascertain when their house would be empty for an extended period of time, and where they live - all from the information shared on socical media. This is why we recommend waiting until you've returned from your trip to post photos or announcements.
The reason sites like these can share this data is because it is either public-record information (i.e, voter registration information, etc), or it is data that has been sold to them from companies you have previously given that data to: this is either a part of the terms and conditions or the privacy policy of the site selling the data. Because of these policies, it is entirely legal for these sites to host this information about you, whether you want them to or not.
However, you can request they take the information off of their site. This doesn't guarantee that it will be deleted and the information will likely still be used for ad-targeting, but it might result in the information being removed from the publicly accessible site in the future. Depending on what site is in question, you can request the removal on the page that hosts the data itself, or try emailing privacy@[companyname].com with a takedown request.
For social media sites that host information about you, consider manually deleting or changing the data that is hosted, especially if you are planning on deleting your account altogether, so you can be sure that the information is removed, or just inacurate.