There are four abstracts method in this class, not three.

src/base_facebook.php

@@ -110,7 +110,7 @@ public function __toString() {
  * Provides access to the Facebook Platform.  This class provides
  * a majority of the functionality needed, but the class is abstract
  * because it is designed to be sub-classed.  The subclass must
- * implement the three abstract methods listed at the bottom of
+ * implement the four abstract methods listed at the bottom of
  * the file.
  *
  * @author Naitik Shah <naitik@facebook.com>