Complex sentence: On December 19,2013, Zuckerberg announced a donation of 18 million Facebook shares to the Silicon Valley Community Foundation, to be executed by the end of the month--based on Facebook's valuation as of then, the shares totaled $990 million in value.
Split into simpler sentences: On December 19,2013, Zuckerberg announced a donation of 18 million Facebook shares, to be executed by the end of the month. Based on Facebook's valuation as of then, the shares totaled $990 million in value.