Google is trying to kill both Apple and Facebook with Android and Google+...
Marissa Mayer is a really smart lady who actually was the key person of the successful of Google(she designed the clean and beautiful home page of Google which directly led Google to success).
She knows that MSFT is trying stealing search market from Yahoo instead of Google with its search deal.
Facebook would need the search technology and patent from Yahoo to fight with Google.
MSFT is out and dying due to its greedy.
Apple wants to build its own search engine, however, it might takes years to archive it.
Before it could do it, Android had already killed IPhone and iPad.
So Apple needs search too and needs search immediately to fight Google.
Both Facebook and Apple has the ambition to fight Google since Google is trying to kill both of them.