Having Facebook know who likes your pages is beneficial in providing you some insight into who is visiting, it doesn't really help Google figure out how awesome your blog post or website are. Yes, Google could probably mooch the Like count on your page; but, that would be uncool in some way.
Having +1 gives Google the same idea about which pages are relevant and popular, which is important when you are running a search, right? I know, it throws SEO under the bus when people get to vote on which web pages are good quality.
Besides giving them metrics on who is visiting what pages, +1 actually helps Google provide their core product, search, much more effectively.