I think it's cute people think social networks (in their current form) will still be around in 18 years. Technology will be in a place that probably very few of us can fathom at this current juncture. I like one of the users' comments on here that the act of registering all of these accounts for their child is akin to registering a geocities name back in the day. We all laugh at it now.
Like I said, the answer is somewhere in the middle. You don't have to do all or nothing. Just be judicious, as you would in your non-digital life (I would hope). Anything in excess can become problematic.
When my partner and I have a child some day, we'll probably be very careful about what we post online. It won't be excessive, but we won't be hermits either as unexisting can be just as odd/problematic as superexisting.