One really cool thing I like about Popfly is the mashups you create can be added to your Facebook profile so that your friends can see and use them. These mashups could be anything from placing your photos on an image carousel to a mashup which places your friends on a map and finds the nearest Starbucks.
Here are some live examples on Facebook right now:
John Montgomery's Halo 3 Scoreboard:
opflymashups/Default.aspx? r=johnmont|Halo3 for Facebook"
Another cool thing you can do is download that Popfly mashup you see on Facebook directly into your own Popfly profile and then tweak it however you like! Brad Abrams showed this off during the keynote at Remix Boston. Download this video off Channel9 and skip to about 16 minutes in to check it out!
Some more links:
PopFly Facebook Mashups Overview:
Main PopFly Page (Now in Open Public Beta)