it doesn't seem so from my experience. typically they were given one email address by their ISP when the account was set up, and that's the one they all use.
it doesn't matter that there are numerous free email providers, or that the ISP is perfectly capable of creating more accounts.. it's beyond the wit of many users to set them up, and/or they don't want to (the parent/kid example again).
only reasonably computer-savvy people seem to create an account for each person.
password recovery is not really the point: most sites these days won't tell you the password, they'll reset it to something random and make you reset it. doing that leaves a trail of evidence if you 'break into' Mom or Dad's facebook account, since the password has changed.
the point is that you can go to any random site that supports this type of login and 'be' the combined family entity referred to by your shared email, since you must all have the password. worse than that, you're FORCED to be the same entity, unless they provide an alternative way of signing up.