"I just bought a used Ford" = "I need a mechanic in six months".
"I'm going rock climbing for the first time this weekend!" = "I need a chiropractor next week".
Etc...
Jokes aside, seems like this would be a fairly approachable mid-way between the existing system and the one proposed.
Like that old commercial pointed out, very few children say they want to "claw my way to middle management" despite the fact that's what happens to a lot of us.
I would expect a service like this to fail partly because people aren't that organized and partly because those that are would be underwhelmed by how things turn out. Nobody likes a service that makes you feel like a failure.
It's pretty simple to see this - few of us even keep our 'formal' calendar up to date, of meetings, calls and plans. Very few people keep their 'informal' calendar up to date. I, for one, never create a task called 'buy truck', 'plan holiday'. The important, high value items never need to be written down. The ones that get tracked and remembered are the low-value ones like 'pay electricity bill' and 'pick up worming tablets for dog'. And they're the ones with little advertising value.
That said, there is probably a specific vertical niche in here that could be expanded, but more of an add-on to an existing network like facebook than a stand alone. Something like a 'holiday planning' where people could post a future plan like a trip to the Maldives, and have their friends (and advertisers) provide input on what to do while you are there.