What should I be aware of with data protection and a user-based website?

Watching Zuckerberg testify in front of Congress is making me think about my recent business idea. It is a social, user-based website that will offer free features (with Ads) and options for premium features. All of this data protection in Europe and recent publicity in America is raising concerns.

What, in particular, do I need to be aware of? Any resources?

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