Social media scraping is an effective passive reconnaissance technique that allows the tester to gather information about a company's employees, technology stacks, and job roles by reviewing publicly available data on various social media platforms. This technique does not involve any direct interaction with the target's systems and can provide a wealth of information useful for crafting targeted attacks or understanding the company's infrastructure. DNS lookups involve querying the DNS servers, which is more about understanding the domain structure rather than employee details. Reviewing open-source code is useful but is more likely to provide information about the technologies used by the company rather than specific user names, unless user information happens to be included in the codebase. Shodan scans would be considered an active reconnaissance technique as it interacts with public-facing devices and could potentially alert the target.