functions.php

<?php
/**
 * When setting up the custom field, set it as a Text Box field type, click on the
 * "Advanced" tab on the left, and enter the pattern below in the "Pattern" input box.
 *
 * This assumes that your meta key for Facebook is "company_facebook" and for instagram, is
 * "company_instagram" ... if it's not, change every instance you see below to match.
 *
 * The patterns below will prevent the user from entering the URL, and only allow usernames
 *
 * Twitter Username Pattern (backwards compatible with old usernames): ^[A-Za-z0-9_]{1,32}$
 * Facebook Username Pattern: ^[a-zd.]{5,}$
 *
 * General only allow alpha numeric: [a-zA-Z0-9]+
 * General username with 2-20 characters: ^[a-zA-Z][a-zA-Z0-9-_.]{1,20}$
 */

add_filter( 'field_editor_output_as_value_company_facebook', 'output_company_facebook_only_show_username', 10, 4 );
add_filter( 'field_editor_output_as_value_company_instagram', 'output_company_instagram_only_show_username', 10, 4 );

add_filter( 'field_editor_output_as_args_company_facebook', 'output_only_show_username_args', 10, 4 );
add_filter( 'field_editor_output_as_args_company_instagram', 'output_only_show_username_args', 10, 4 );

/**
 * Remove URL and only output Username as caption
 *
 * Because our filter for the value is before the arguments, we need to remove
 * the URL from the value, and then set the caption as that value (with URL removed).
 *
 * @param $args
 * @param $value
 * @param $listing_id
 *
 * @return array
 */
function output_only_show_username_args( $args, $value, $listing_id ){

	if( ! is_array( $args ) ) return $args;

	$username = str_replace( 'http://instagram.com/', '', $value );
	$username = str_replace( 'http://facebook.com/', '', $username );

	$args[ 'output_caption' ] = "@{$username}";

	return $args;
}

/**
 * Add Facebook URL to field value
 *
 *
 * @param $value
 * @param $slug
 * @param $listing_id
 * @param $args
 *
 * @return string
 */
function output_company_facebook_only_show_username( $value, $slug, $listing_id, $args ){

	if( ! empty( $value ) ){
		$value = 'http://facebook.com/' . $value;
	}

	return $value;
}

/**
 * Add Instagram URL to field value
 *
 *
 * @param $value
 * @param $slug
 * @param $listing_id
 * @param $args
 *
 * @return string
 */
function output_company_instagram_only_show_username( $value, $slug, $listing_id, $args ){

	if( ! empty( $value ) ){
		$value = 'http://instagram.com/' . $value;
	}

	return $value;
}