Is there a way to add a link (Lets say a facebook page) to the main navigation at the top?

Yep, certainly. Create a new menu item using the Menu Manager in the ACP: System > Menu Manager. Select "External Link" for the new item which you're creating.

For 3.4.9 open your globalTemplate and find:

							<li class='right'>
								<a href="{parse url="app=core&amp;module=global&amp;section=navigation&amp;inapp={parse expression="IPS_APP_COMPONENT"}" base="public"}" rel="quickNavigation" accesskey='9' id='quickNavLaunch' title='{$this->lang->words['launch_quicknav']}'><span>&nbsp;</span></a>
							</li>

Add below:

<li id='nav_other_apps'>
<a href='https://www.google.com/' target='_blank' title='New Tab'><div class='umiesc'>New Tab</div></a>
</li>

If you want to hide the tab for separate groups add:

<if test="!in_array( $this->memberData['member_group_id'], array( 1,2,3,4,5 ) )">
<li id='nav_other_apps'>
<a href='https://www.google.com/' target='_blank' title='New Tab'><div class='umiesc'>New Tab</div></a>
</li>
</if>

Change the array( 1,2,3,4,5 ) to the groups ID that you want to hide the tab.

Or see the Pro Menu solution