If things like hi5 and myspace - in my opinion - much worst than facebook, are still going strong, I can't see how facebook won't continue to be relevant for at least few more years

I kind of share your same observation, most of my friends dont use facebook as much, but they still do, they check it like email to read all the messages, know about outings, know about new events (concerts, bdays, etc .... )

So to your observation I add, its possible that the first wave of facebook users ( i will call them the addicts ) have faded away

But now there is the second wave ... the casual users wave which is probably just as big, only different!

Finally if facebook post site usage statistics, this should be a more concrete indication of how well the site is doing