Just had a similar ~30% drop over the weekend, with no SE percentages significantly changed (if anything, G's % has increased and that normally converts into more traffic, not less).
Poking around and playing with my own smart phone, it looks like there has been some kind of a trouble with IPv6->IPv4 conversion at Verizon Wireless gateways over the weekend and none of non-IPv6 enabled servers could be reached. I had my mobile traffic halved this weekend, whereas usually on weekends mobile share grows to over 50% or the overall traffic. Was kinda surprised to see how big of a share VZ has - almost half of all my mobile traffic from VZ and all of it was lost over the weekend. It has just cleared about two hours ago, so chances are, Monday will also be pretty lousy.
The whole episode made me appreciate importance of mobile traffic for my sites. I kind of kept it always on the back of my mind but this was nothing subtle at all, definitely a major factor now.