In America at least, black clothing (typically formal wear, the specifics vary from place to place) is strongly associated with mourning. It's traditional garb at funerals and gatherings to pay respects for the departed. This may be different in other countries, and it's not a universal practice, but it's common enough that most everyone understands the meaning.
For context for those who are not already aware, since this is being downvoted... white traditionally has the same associations in Chinese culture. So it was not an entirely unreasonable question. The answer is just that this happened in the US, not China, and wearing white in the US would be unusual.
This is a pressure tactic against Facebook. They are also carrying signs and chanting slogans. Of course the signs and slogans are in English; wouldn't do much good in Chinese. So it makes sense they would follow the custom of the country they are in and the company they are trying to pressure.
