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.
[0]: http://s1.q4cdn.com/568883301/files/doc_financials/2015Annua... - page 66
[1]: http://s1.q4cdn.com/568883301/files/doc_financials/2015Annua... - pages 59 & 64