Presumably the class C shares would have rights to a dividend and not to vote. This would let Zuckerburg diversify his investments without giving up more control of the company.
It's interesting that FB will be initiating dividends - I am no expert when companies usually start offering dividends but I was under the impression it's usually further in time when companies have been in business for a while. FB is still growing a lot so I am surprised - but again no expert in that.
I'm assuming that the details of the "capital" shares would include them receiving dividends, but this doesn't mean that Facebook would necessarily start paying a dividend, it's just a right that has at least some value.