Only Yelp can say, but I think that Google's acquisition strategy seems to pretty clear: "If we can't buy you, we are going to buy one of your competitors." The questions you'll need to ask yourself when Google comes knocking are:

1. How much do I respect my competitors? 2. If Google does get them, will they do better or worse?

It's not a matter of hoping Google will go away. Once they've signaled intent to be a part of your business, you can be sure they will, one way or another.

I thought it was "only" $500M, but regardless, they're clearly gunning for an IPO. Hard to say how that will pan out, though. LinkedIn has an $8B market cap, but Groupon's IPO seems stalled.