Even if they start asking for passwords, you can almost certainly rely on them not considering the impact of 2-factor. You could give them the password knowing full well that without confiscating your phone for the duration of the trip they can't get into the account. Plus what is to stop you changing the password once you get through border control, or just denying you even have a social media account, suspend it temporarily before travelling.
This seems to me just another excuse under the guise of anti-terrorism to pry into people's private lives.