Yahoo is doomed, and has been for years. It's a geriatric patient ticking down to heart attack. Google will always be a search engine, and as much as it innovates, it'll be a search engine before it's a lifestyle.
As for M$, Well, They'll be in email, chat and interfacing as long as people buy Windows.
To put it in Media terms:
Microsoft is Viacomm
Google is the New York Times
Yahoo! Is every local news paper out there.