As individuals, we don't act the same way around our peers, our friends or our family. While we certainly present ourselves differently in different contexts, I wouldn't call these different "identities", rather the same identity applying the rules of the given context to their behavior.

Where it becomes tricky is when one is strips away the context isolation. For example, at your friend's bachelor party, some behavior might perfectly normative within context and participants, but the next day when people outside of that context can watch a video of your behavior - you might feel a little shame/embarrassment/regret/etc.

The internet, for better or worse, allows a complete breakdown of context isolation. While the rules of posting on 4chan or HN greatly differ, a person adhering to rules of a given context, will still be judged across both contexts by outside viewers.

The solution here is to create context isolation - which is very easy to do with anonymity.