All it takes is for Facebook to go from $4 a user/yr to $8, or for teenagers to say "email me, I don't check facebook often". A powerful coil indeed.
Facebook exists because I would not like 500 friends to email me every day about their photos or jokes. That sort of thing doesn't scale well with emails.
They have now become a tax on life - like email. A set of hygiene actions you have to perform every day.
Do not underestimate how much the next generation does not want to conform to the restrictions of the previous generation.
It's entirely possible future generations do not want to be tracked and targeted 24/7 - they may choose to just switch off and drop out.
50% youth unemployment in some first world countries right now. These guys may just decide to do things differently - what is social technology doing for them right now?
You're dead right that each generation wants to be in a social space that feels "theirs", but I don't think that precludes using the same infrastructure.
When I was a teen, I drove on the same roads and used the same telco and USPS as my parents, even at my most rebellious times. In Facebook's case, each user "feels" a tiny piece of the overall universe, and one which is vastly disproportionately populated with people they've mutually selected. That, coupled with meeting jwz's use case above, makes me feel that Facebook is pretty well positioned to succeed.
Those industries exist because of the government monopoly, a new better social network can arise in an instant.