Can a Facebook app possibly be useful?The way people talk publicly about Facebook's application platform, you'd think Jesus used it to invite his apostles to the Last Supper. But some industry insiders quietly say they're not at all impressed with the applications people have developed. "None of the most popular apps actually do anything," a high-ranking Yahoo developer recently told Valleywag. It's hard to disagree: "Top Friends"? "Food Fight"? But Facebook's cornucopia of uselessness may gain some measure of utility on Friday. Friendvox, a new Facebook instant-messaging tool, is going into beta then, according to Blognation UK. It turns your roster of friends, instantly, into a buddy list, and lets you exchange messages without the tedious back-and-forth of Facebook's built-in email system. It's a great idea — so great that we're sure that Facebook will, in short order, steal this idea for itself and build IM functions into the site.