$100b a bargain, I'm not so sure. I'd be more inclined to describe it as a fair valuation.
This is the real reason Google is getting desperate about Google+, they are worried about FB launching its own hyper targeted ad network. Google has context and tons of advertisers, FB has context and deep demographics and quite a lot of advertisers.
I think it's more that Google wants in on that demographically targeted ad-game. Right now search ads and facebook display ads are really different beasts.
For instance, in my business Facebook ads are worthless. We depend on user intent "I'm looking for THIS" (in our case local service professionals). In search that intent is very clear, in Facebook way less so.
If I was selling lifestyle merchandise or advertising a consumer site, Facebook would be really attractive I'd think.
Both sides want a big piece of what the other has.
This is the real reason Google is getting desperate about Google+, they are worried about FB launching its own hyper targeted ad network. Google has context and tons of advertisers, FB has context and deep demographics and quite a lot of advertisers.