That's the best option, but it rarely happens. Usually rivals simply fight it out. Remember Sun and Netscape? In the 90s the two could have collaborated to fight Microsoft on the browser front, instead they fought bitterly. Netscape died, Sun was mortally wounded, and IE ruled (still does).
Google might be smarter with OpenSocial:
BBC NEWS | Technology | Google opens up social networking:I bolded LinkedIn because it's the social face for one's corporate self. A unique front with few challengers. I'd expect Google to invest in them if they haven't already.
...Google said that around a dozen social network partners had signed up to the system, including business site LinkedIn, Friendster and Google's own social network Orkut...