sorry, i don't know what popular forums in uk and us, cause i don't live there
about google and yahoo, they have their own strongness and weakness. google is the fastest search engine, and their mail service (gmail) which capacity is 2 GB is followed by their competitor like yahoomail (which have just improve their free mail to 1 GB) and so on
yahoo is a legend in web, cause they have so many services to make people comfort, like mail (i used it since i'm junior high!), webhosting, groups (i follow about 20 yahoo groups), briefcase, 360 (like friendster), movie, sports, games, and many more..!
so if u search a forum, consider with ur interest, but friendster is still a fenomena until now
cmiiw