I think whether it is sticky or not is not the right item to focus on (though it obviously is). The question of whether or not they have serious user churn is the interesting one.
I don't believe it is and here's why. It's easy to link to your facebook profile. If you create an orkut profile, you can add a link to your facebook profile there and simply stop using facebook.
Eventually, it'll get too costly to maintain the infrastructure to support facebook or the customers -- that is, the advertisers -- will think another ad network is hotter and people will forget about facebook.
You could say, "But all my messages are in there. All my photos and my farm..."
But I'm a techie and I have had computer hard drives fail and take my life with them. Within a week, I forgot all about it. Sure, I wish I had that awesome paper I wrote for some class or another or the code for some program I wrote in high school that drew pretty pictures with mathematical formulas, but I'm okay with out it and life goes on.
And life will go on without facebook. Life goes on without myspace and linked in and friendster and...
