function bp_activity_add_user_favorite( $activity_id, $user_id = 0 ) {

	// Fallback to logged in user if no user_id is passed.
	if ( empty( $user_id ) ) {
		$user_id = bp_loggedin_user_id();
	}

	$my_favs = bp_get_user_meta( $user_id, 'bp_favorite_activities', true );
	if ( empty( $my_favs ) || ! is_array( $my_favs ) ) {
		$my_favs = array();
	}

	// Bail if the user has already favorited this activity item.
	if ( in_array( $activity_id, $my_favs ) ) {
		return false;
	}

	// Add to user's favorites.
	$my_favs[] = $activity_id;

	// Update the total number of users who have favorited this activity.
	$fav_count = bp_activity_get_meta( $activity_id, 'favorite_count' );
	$fav_count = !empty( $fav_count ) ? (int) $fav_count + 1 : 1;

	// Update user meta.
	bp_update_user_meta( $user_id, 'bp_favorite_activities', $my_favs );

	// Update activity meta counts.
	if ( bp_activity_update_meta( $activity_id, 'favorite_count', $fav_count ) ) {

		/**
		 * Fires if bp_activity_update_meta() for favorite_count is successful and before returning a true value for success.
		 *
		 * @since 1.2.1
		 *
		 * @param int $activity_id ID of the activity item being favorited.
		 * @param int $user_id     ID of the user doing the favoriting.
		 */
		do_action( 'bp_activity_add_user_favorite', $activity_id, $user_id );

		// Success.
		return true;

	// Saving meta was unsuccessful for an unknown reason.
	} else {

		/**
		 * Fires if bp_activity_update_meta() for favorite_count is unsuccessful and before returning a false value for failure.
		 *
		 * @since 1.5.0
		 *
		 * @param int $activity_id ID of the activity item being favorited.
		 * @param int $user_id     ID of the user doing the favoriting.
		 */
		do_action( 'bp_activity_add_user_favorite_fail', $activity_id, $user_id );

		return false;
	}
}