Once personal data is shared, it should be considered to no longer be personal, regardless of what the other party claims they wish to use it for.
Data is one of the biggest markets and social platforms like Twitter, Facebook, Google and the rest have loads of it by collecting presumably harmless data to fulfil whatever sign up conditions they set. It has become so much of a norm now, users barely care what is collected and how it is used, or rather they fail to see any danger in their phone number, or web history being tracked and stored.
This is a trend that is not going to stop anytime soon, best to take precautions to protect yourself if you do not want your details shared.
Unfortunately, there are companies making billions off this situation and they are fine with it. Remember that before 2016 elections of trump, and even brexit, there were companies buying and selling data and using it to make sure that they got their candidate elected.
If they could convince their own view people to go out to vote in bunches, they did that via ads, if they could convince opponents people to stay home, they did that. They did everything they could, not just to convince people to vote for a candidate, but precise ads for each person in order to get the most optimum result for their candidate. This shows that these data could be used to literally rule the world.