One thing: it’s interesting that your view is based from the advertisement perspective, ie. collecting eyeballs for (targeted) adds.
In my opinion social networks will add value not to collect eyeballs but to supply valuable services, content or even third party products.
In that respect it’s not a matter of large networks sharing identities – as the identity is owned by the person, but a identity -contextualized by metadata, friends and content- traversing a sea of services. Like you trust persons to be your friend in social networks, you’ll trust service providers (now known as social networks; like flickr, delicious, digg) to be your provider of valuable content (banners, products, online services, access).
The big social networks are in not endangered by FSN, but are offered a brand new opportunity to monetize their services.