Google has no choice but to take on Facebook. The reason is simple: facebook generates (commercially) valuable data and its popularity is soaring. As seen in this Google Insights plot an enormous portion of all the world wide searches which contain the word login also contain the word facebook (in blue all “login” seaches; in red all “login” searches which are not about facebook!).
Most likely these are so called navigational searches where instead of typing the URL you are interested in you search for it in Google and then click on one of the top links which is what you need. There is no other token which makes such a big part of the login searches and in fact the combination of “google gmail googlemail orkut” make up just a tiny fraction of the login searches. This is why Google has no option but to up the bets to Google+.