This chart is buzzing. According to online traffic tracker comScore, Facebook is now serving up nearly a quarter of all online ad impressions. That’s interesting, but it doesn’t make Facebook the king of all (online) media. Heck, Myspace once held a similar position. Facebook is generating a lot of what I got in trouble for calling onstage “crap inventory.” That is, largely undesirable space to show cheap ads – best used for direct marketing that only gets paid for if converted. Most of these impressions don’t tap the potential of social media: e.g., giving advertisers a chance to engage with their prospects, harness viral pass-along and personal recommendations, or get real-time feedback on campaigns or products. Facebook’s working on all that, but these stats don’t necessarily indicate progress.