They keep their facebook account, until they don't. Large numbers of people have opted out of facebook in recent years. Just because it's been around for a few years, doesn't mean it is permanent. Nobody is finding me on MySpace to stay in touch (at least not in the last decade...and even then it took me a year to see the message because I hadn't logged in in longer than that), and while facebook has proven to be longer lasting than MySpace, we don't have enough evidence to say they'll be relevant in ten or twenty more years.

Also, I've had the same cell phone number for approximately 20 years (and I only changed the number then because I wanted one with an area code in the city I was doing business in...which seems like a quaint requirement today). I am legally allowed to take my number with me when I change carriers. I have no legal right, or technical means, to carry my facebook profile URL with me if I move somewhere else. I hate phones, but if we're talking about longevity of connectivity...phones do alright compared to social networks.

> Large numbers of people have opted out of facebook in recent years

It is still a minority in my personal experience (which is what matters for me).

> Also, I've had the same cell phone number for approximately 20 years

I think you're probably a few generations above me, probably none of my friends still have the same phone number, I know I've changed a dozen of times in the last 10 years. It's easy to keep the same cellphone number if you live all your life in the same country, but that's not my life or my friends life.