Zuckerberg donated to Diaspora, the open-source Facebook competitor
My take is that Zuckerberg doesn't feel threatened by Diaspora, and that he might actually see it as an incubator for new ideas that Facebook can use to push its agenda of opening the Web. [See Microsoft bailing out Apple back in 1997! -Ed] It also wouldn't be surprising to see Zuckerberg make an attempt to hire the Diaspora team at some point, although I doubt they'd be swayed by the offer. Including Zuckerberg's donation, Diaspora has raised nearly $200,000 on Kickstarter.
For more on Diaspora, check out Download Squad's point-counterpoint series: Why Diaspora Will Win and Why Diaspora Will Fail.