Well, you can find out for yourself. On a daily basis, those neat people at Programmable Web scour the earth for mashups, and figure out which APIs are being used in them. Then, they tally who's using what to figure out which web services are the most popular, and for what.
Yahoo leads in APIs, but google is running away in Mashups. The most popular APIs?
1) Google Maps
2) Flickr Photo Sharing
3) Amazon Shopping
Exactly 12 mashups using Voice, with 8 of them using Skype from Ebay.