infinite_scroll_credit filter, like so:
function tweakjp_cust_credit() {
return '<a href="http://example.com">My Custom Link</a>';
}
add_filter( 'infinite_scroll_credit', 'tweakjp_cust_credit' );
Once you add that code to your theme’s functions.php, a new link will appear instead of the default “Powered by WordPress” link. 
Jetpack 