1. It's not a visa but an ESTA. The visa is still granted on the fly on entry.
2. The U.S. require the same the other way around, only the one granted by the EU is $10 cheaper and valid for 3 years instead of 2, so still U.S. citizens get an advantage
3. EU citizens (like all other non-immigrants) have to, as far as I understand, disclose all their social media accounts when applying for a US visa

Sources for (3):

For VISA applications, https://travel.state.gov/content/dam/visas/Enhanced%20Vetting/CA%20-%20FAQs%20on%20Social%20Media%20Collection%20-%206-4-2019%20(v.2).pdf should apply.

What if applicants participate in multiple online platforms? Are they being asked to list all of their handles, or only one?

Applicants must provide all identifiers used for all listed platforms.

I reached that document via https://www.ustraveldocs.com/de/de-gen-faq.asp#qlistgen21 ("Apply for a U.S. Visa in Germany") and didn't find any hint for exemptions for German citizens or E U citizens, so I assume it applies. (But I might still be wrong.)