the_permalink on the homepage vs posts for sharing links

I would turn this into a function to account for all conditions. Take the following:

function generate_facebook_sharer_link( $label = 'Share this on Facebook', $classes = '', $return = false ){
    if( is_singular() ){
        // If is a Single CPT, Post, Page, etc.
        $url   = get_permalink();
        $title = get_the_title();
    } else if( is_front_page() ){
        // If this is the "Main Page"
        $url   = home_url();
        $title = get_bloginfo( 'title' );
    } else {
        // All other Accounts (Archives, Blog Page, etc.)
        $url   = ( is_ssl() ? 'https://' : 'http://') . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];
        $title = get_bloginfo( 'title' );
    }

    $output = sprintf( '<a class="%s" href="http://www.facebook.com/sharer.php?u=%s&t=%s" target="_blank" rel="noopener noreferrer">%s</a>', $classes, urlencode( $url ), urlencode( $title ), $label );

    // Allow returning, default to echoing
    if( $return == true )
        return $output;

    echo $output;
}

This will output a link wherever generate_facebook_sharer_link(); is called. The is_singular() should catch all Custom Post Types, Posts, Pages, etc. is_front_page() will catch if it's the "main page", regardless of what the Reading Settings/Blog Page are, and then it falls back to building the current URI with appropriate protocol and setting the blog title.

This could also very simply be turned into a shortcode:

add_shortcode( 'facebook_share_link', function( $atts ){
    // Extract and Define Defaults
    extract( shortcode_atts( array(
        'label'   => 'Share this on Facebook',
        'classes' => '',
    ), $atts ) );

    if( is_singular() ){
        // If is a Single CPT, Post, Page, etc.
        $url   = get_permalink();
        $title = get_the_title();
    } else if( is_front_page() ){
        // If this is the "Main Page"
        $url   = home_url();
        $title = get_bloginfo( 'title' );
    } else {
        // All other Accounts (Archives, Blog Page, etc.)
        $url   = ( is_ssl() ? 'https://' : 'http://') . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];
        $title = get_bloginfo( 'title' );
    }

    $output = sprintf( '<a class="%s" href="http://www.facebook.com/sharer.php?u=%s&t=%s" target="_blank" rel="noopener noreferrer">%s</a>', $classes, urlencode( $url ), urlencode( $title ), $label );

    return $output;
} );

Which would allow you to plop [facebook_share_link] wherever you'd like it to show up.

This code displays takes get_permalink() on a single page and get_home_url() on the homepage. If a link is found the Share-Link will be displayed.

So the Link wouldn't be displayed right now on a category-page for example.

<?php
    if( is_home() ){
        $link = urlencode( get_home_url() );
        $title = urlencode( get_bloginfo( 'title' ) );
    }elseif( is_single() || is_page() ){
        $link = urlencode( get_permalink() );
        $title = urlencode( get_the_title() );
    }

    if( isset( $link ) ):
?>
<a href="http://www.facebook.com/sharer.php?u=<?php 
    echo $link;?>&amp;t=<?php echo $title; 
?>" target="_blank">Share this on Facebook</a>
<?php
    endif;
?>

Info:

• get_home_url(): https://codex.wordpress.org/Function_Reference/get_home_url
• is_home(): https://codex.wordpress.org/Function_Reference/is_home
• is_single(): https://codex.wordpress.org/Function_Reference/is_single