Why? Context (both situational and by social norm) and intended use. People change their behavior based on a lot of factors; they choose what they disclose and

Why? Context (both situational and by social norm) and intended use. People change their behavior based on a lot of factors; they choose what they disclose and modify that choice in realtime as the context changes. For example, two people talking on the street are clearly in public. However, if they look around and see that there isn't anybody near them, nor are there any recording devices[1], they may choose to speak in confidence. While they are technically "in public", they are de facto private, at least temporarily. Pivacy isn't a Boolean property, and it can change all the time. Expanding on the same example, if there are three people in the same situation, something said between Alice and Bob is obviously not "private" because Carol can overhear it, but there may be an expectation that it is public within that group, but private with respect to everyone else. For OkCupid, I would guess that most people have an expectation that the data they share "publicly" will be used in the expected features of the site. Their data won't be public in the general sense, but it will be public to people with OkCupid accounts. Many people also expect that they data will be used[2] as the site presents the data being used. Searches done manually are different than a program that walks the entire database. Anybody that wants to work with technically public data in a way that is outside the usual user-available features should really be getting informed consent first. This has nothing to do with legal restrictions (which may also exist in some situations); failure to ask first will simply earn you a reputation of being a "peeping tom" that shouldn't be trusted. [1] see the recent situation in Oakland with the FBI hiding microphones [2] modulo minor administrative uses done by OkCupid itself