I can't wait for entrepreneurs to take on LinkedIn. There are a lot of opportunities there.
There's so much untapped potential to build upon aggregated markets (of professionals, of jobs, of housing, of singles, of restaurants, etc) rather than building yet another aggregator. The existing aggregator cements its dominant position, can charge rent to connected apps, and attracts rather than repels killer apps.
The main problem I see is fears that connected apps will siphon away the aggregator's data, and/or combine that data with external data; both are attempts to supplant the aggregator. Maybe this fear can be assuaged by technical or legal means.
We need DE-centralization, so no-one in the world has this type of power over anyone.