If you’re trying out Twitter’s Web Intents Javascript Events and it’s not working, be aware that they only work if your HTML page is loaded from an HTTP server. If you open your page locally off your hard drive it won’t work, though you can fire up an Apache instance on your localhost and it’ll work from there.
Source: https://dev.twitter.com/comment/reply/304/491
Update: It looks like this can also happen with the Facebook JavaScript API: http://forum.developers.facebook.net/viewtopic.php?pid=362167#p362167. I guess it’s because they use iFrames, and using them locally can cause security problems.
Seán Hayes 