Speaking in terms of actual visitors, the China Pavilion – which reached 1 million visitors in the first 13 days of the Expo, is way ahead of the rest of the pack which only started hitting the million count around the 1 month mark.
If you look at which pavilion is generating the most interest on the internet, there’s no easy answer. Break it up into segments and sites, and it gets more interesting.

Seed Cathedral, UK Pavilion, Shanghai World Expo
1. Flickr – Which pavilion’s photos are being uploaded and shared on sites like Flickr and Twitter the most number of times? The clear winner is the UK Pavilion. The Seed Cathedral is by far the most popular Expo pavilion picture on the internet, and the craze even extends to video (see YouTube below).
2. Foursquare – Of all the country pavilions at the Expo listed as venues on Foursquare, the USA Pavilion got the most number of checkins (167 as of June 12), followed by the China Pavilion (65), and UK/Japan tied for third place with 47 checkins each.
Also to be noted that the North Korea Pavilion has 32 checkins, which makes it one of the leading Expo country pavilions on Foursquare, just behind Canada (33).
3. Social Media - Only a handful of Country Pavilions have official Twitter accounts or Facebook pages. Out of those who did create an account, the USA Pavilion leads the game on Twitter (@MeiGuoGuan) with 2,466 followers as of June 12. The USA Pavilion also has created official accounts on Sina, QQ, Renren, Flickr and Youku.
4. YouTube – The most popular Expo country pavilion videos uploaded in the last 2 months are both of the UK Pavilion – one with over 53,000 views (as of June 12) and the other with 41, 108 views. The second most popular Expo pavilion on YouTube is the Japanese Pavilion, where the violin-playing robot from Toyota is attracting a lot of attention.
Upcoming YouTube expo videos of specific pavilions include the Australia Pavilion’s Rusty Shed and the wind tunnel Aerodium at the Latvia Pavilion.
In summary – On the internet, the UK Pavilion is the most visible pavilion, with the USA Pavilion most popular on social networks.
Photo – Gabyu

not a fair gauge of popularity
obviously the number of people who are into social media cannot be proportionally equivalent to a pavillion’s popularity.
Hi dp,
Of course, that’s why P Ling said “on the net”. ;-)