Some pay with points that are redeemable for cash or goods and typically you must rack up a bunch to redeem them for anything of significance. Others may give you a gift card, discount, or another token of appreciation for participating. Many others pay nothing or only offer sweepstakes entries for completing screening surveys to determine your eligibility for other, paid surveys. A few don't pay much of anything, unless you recruit others, as in a pyramid scheme.
Most reviewed paid survey sites effectively promise not to share personally identifiable information or not to share it without your consent. It's an industry standard by which legitimate marketing research firms are bound. But many membership sites reviewed don't make either promise or do so only in a limited or wishy-washy way. Unauthorized go-betweens don't have to honor marketing research privacy standards.