because I’m not echoing anything in this code.
But you are! This code:
function smb_function( $atts ) {
?> <div class="social_button_shortcode"><?php
Is the same as:
function smb_function( $atts ) {
echo ' <div class="social_button_shortcode">';
Likewise with the closing div tag at the end. Any form of direct output in a shortcode will break it, wether it was done via echo, print, printf, or other WP functions.
The shortcode also breaks the rule that shortcodes should return their content, because it only returns when if ( count($smb_url) > 0 ) { is true. But what if it isn’t true? And notice that the function can never reach the closing div tag because of the return.
As an example, what would the output of this shortcode be:
add_shortcode( 'blank', function() {} );
It’s equivalent to this:
add_shortcode( 'blank', function() { return NULL; } );
And this:
add_shortcode( 'blank', function() { echo "Hello World!"; return NULL; } );
All 3 shortcodes produce a null value.
If you absolutely must output directly, use an output buffer to catch it and assign the result to a variable.
tip: typehint the shortcode function as returning a string, it’ll make it more obvious if you don’t return a string