The question says quote “Select from the list below each social media platform you have used within the last five years. In the space next to that platform’s name, enter the username or handle you have used on that platform.”
The question mentions not to enter your password. Because of this, it is not likely an officer will see private content initially. However, CBP agents have been known to have you pull up your email or social media accounts on your phone and hand it over to them for review during entry to the US. The significance of this social media question cannot be understated. Not only will they be looking for the obvious terrorists or drug smugglers, but they are likely to also use your social media content to discover inconsistencies with your story and to catch you in a lie.
For example, if you have Instagram photos announcing how you’re moving to the USA to marry your girlfriend, but you are applying for a B1/B2 visa and pretending to come on a vacation, this would be considered immigration fraud and can be used to not only block you, but ban you.
As we always warn people, be cautious of what you’re posting to social media. If you don’t want it to be seen by an immigration officer, it’s best to keep it off social media. It’s also important not to withhold any social media profiles from the DS-160, as any lie that you’re caught in can and will be used against you. So if you have an Instagram account, but you lie on the DS-160, and they find an Instagram account on your phone when you enter the border, that would be you being caught in a lie, and you can be turned away and banned from future entry. It’s not worth it.