Index: bp-members/bp-members-functions.php
===================================================================
--- bp-members/bp-members-functions.php	(revision 6195)
+++ bp-members/bp-members-functions.php	(working copy)
@@ -504,7 +504,7 @@ function bp_core_get_active_member_count() {
 		$exclude_users = $wpdb->get_col( $sql );
 		$exclude_users_sql = !empty( $exclude_users ) ? $wpdb->prepare( "AND user_id NOT IN (" . implode( ',', wp_parse_id_list( $exclude_users ) ) . ")" ) : '';
 
-		$count = (int) $wpdb->get_var( $wpdb->prepare( "SELECT COUNT(user_id) FROM $wpdb->usermeta WHERE meta_key = 'last_activity' {$exclude_users_sql}" ) );
+		$count = (int) $wpdb->get_var( $wpdb->prepare( "SELECT COUNT(user_id) FROM $wpdb->usermeta WHERE meta_key = %s {$exclude_users_sql}" ), bp_get_user_meta_key( 'last_activity' ) );
 		set_transient( 'bp_active_member_count', $count );
 	}