Hi, I heard a lot of similar questions about drupal performance. Can agree that we are not able to create facebook using drupal, should mention that even pure php is not very suitable for this purpose. (As you know facebook created own php extension which compiles php code in c and execute it faster then native php interpreter.) Drupal is disigned for sites which have mostly anonymous users as main audience. There are sites like examiner.com which designed for logged in users. Such sites use drupal with mongo as db engine, but there are not too many of them.
Sometimes we need to provide php profiling to find bottlenecks inside our site. Xhprof extension is one of the best solutions. Here is example of found small issue and given graphs before and after chnages.
Graph before changes
After changes (I just added module_load_include into condition to call it not so often).