Google hiring at a massive rate isn't necessarily going to be their downfall although it'll be interesting to see if bureaucracy rears its ugly head. Startups beyond the IPO stage usually do as they become more established. If anything, it's if their stock value goes down (it has for a while) because of two things: 1) someone figured out something that could possibly disrupt Google's search (it might be Facebook) in the way search activities are done and/or 2) they can't find more inventory to sell ads against. There's not a ton of added value on selling Adwords on Google or any other affiliate site. Google is slowly integrating ad products against Youtube videos and search results but they're not pressed for time either, which one would think should last them a good while. As a result, Google would probably have to keep acquiring in order to keep growing or create new products to sell advertising against in some way. HR expenses can bog Google but given the war chest they have, why not, although they're kind of in a Catch 22 situation of sorts.
It's kinda like the Old Cafe - http://fbc2.freeforums.net