I've no idea. Is utterly meaningless. A more meaningful comparison would be "what $ value has been transferred using each network over the last 12 months"
A more meaningful comparison would be the market cap of various bitcoin exchanges; the flaws with that comparison are pretty much the same as the flaws with comparing bitcoin to WU in general.
The best data I could find on this is in their 2015 annual report, which says they've had $81.6 billion in consumer-to-consumer transfers[0], which made up 79.21% of their revenue[1], so I guess we can estimate an annual turnover of $103 billion, which comes out to $282 million per day over the year of 2015.
Nope, I'd assume that B2B has lower fees/margins, so the actual daily volume should be higher than my calculation said above. Still, it should be enough to establish that WU is still a fair bit ahead of BTC.
Doesn't that by necessity include all the change as well? So it's a misleading number if you want to take it for "this is how much people are paying each other with bitcoin"
Keep in mind though that anyone with reason, could easily fake a lot of volume: 2 address, starting amount, swap back and forth until you eat up the whole thing with fees.