Can i build a Social Networking site

depending on your definition of "simple" the answer is yes. There are many modules in the downloads area that allows you to expand drupal core to offer your visitors a social networking experience. However, to customize stuff, will require some learning.

No you don't need to know ANY code, or really anything at all about websites, to make a Drupal site. Its *highly* recommended though that you understand coding concepts (such as arrays, includes, strings, etc). Obviously, total coding knowledge will help you alot, but its not needed.

I was in the same boat as you a few months ago. I had been out of the CMS/CMF loop for years and began to benchmark them against one another. My goal was simple - i needed a system that is completely scalable and stable from the framework up, and i needed modules that are consistently awesome (and stable/scalable hehe). I started with Joomla and Wordpress, but quickly realized their limitations. Moved onto many others and the only ones that were really impressive were Plone and Drupal. Plone is basically a monster suited for enterprise, research, huge networks, etc (its also pretty damn complex and uses python) so i opted out. This left me at Drupal, and I'm sorry i waited until the end of the list to test it!

My first Drupal site was just a local test using Wamp (you can also use Xampp). I wanted to make a social network with friends, forum, ratings, and a wiki. Seems pretty simple right? Well, for the most part it is! You just have to understand the terminology and admin areas that Drupal uses. Id REALLY recommend the book "Drupal 6 Social Networking". I dunno who sells it, but I was able to easily follow it. It took about 2 days to get through it. After, I understood almost all of Drupals innards and how to do many many things. Here are the modules ive worked with to make a more-than-basic social network site. Some of these are used on Drupal.org and many others. The caps ones are "most important":

ACTIVETAGS (BETTER FREE TAGGING)
ADDRESSES (CONSISTANT ADDRESSES + CONTRIES LIST)
ADMIN_MENU (SWEET ADMIN MENU/TASKBAR)
ADVANCED_FORUM (TURNS FORUM INTO SOMETHING LIKE PHPBB/SIMPLEMACHINES)
BACKUP_MIGRATE (EASIER BACKUP AND MIGRATION)
BETTER_PERMS (PERMISSIONS LIST ITEMS ARE COLLAPSABLE)
calander (views calander)
CAPCTCHA (spam prevent)
CCK (CONTROL OF CONTENT TYPES AND DATA ENTRY)
date (set dates, uses javascript calander)
DEVEL (GOOD DEVELOPMENT FEATURES)
DHTML_MENU (BETTER MENUS)
DIFF (COMPARE REVISIONS)
ext_link_page (show user a page when they are leaving site)
facebook_status (status messages)
FILEFIELD (FILE UPLOAD ABILITY)
FIVESTAR (VOTE/RATING SYSTEM)
FIVESTAR_EXTRA (EXTRA GOODIES)
FLAG (CUSTOM FLAGGING, LIKE BOOKMARKS)
GUESTBOOK (USER WALL)
IMAGEAPI (NEEDED FOR IMAGE HANDLING)
IMAGECACHE (IMAGE MANIPULATION)
imagefield (uses file field to attach image)
imce (file browser)
imce_crop (croppa)
imce_wysiwyg (file browser WYSIWYG plugin)
JQUERY_PLUGIN (SWEET JAVASCRIPT LIBRARY)
JQUERY_UI (INC JQUERY.UI PLUGIN)
JQUERY_UPDATE (ALLOWS DRUPAL TO USE NEW JQUERY)
lightbox2 (popup with dark bg, ie. displaying an image)
LOGINTOBAGGIN (BETTER LOGINS)
MESSAGING (MESSAGING SYSTEM)
NODE_ACCESS (GRAINULAR CONTROL OF WHO SEES WHAT)
node_expire (content expiration)
NODEQUEUE (CAN ADD CONTENT TO SHARED LISTS, IE. SANDBOX)
noderef (attach/reference other content)
noderef_url
notifications (notifications and subscriptions system)
OG (GROUP SYSTEM)
og_contact (contact page for groups)
OG_FORUM (GIVE EACH GROUP A FORUM)
PATHAUTO (CHANGES URL TO TITLE OF CONTENT)
PRIVATEMSG (MESSAGING SYSTEM PLUGIN, USER TO USER PRIVATE MESSAGING)
remember_me (adds remember me box in login)
role_change_notify (tells user when their role changes)
role_limits (set caps on number of users in roles)
RULES (MAKE YOUR OWN ACTIONS AND TRIGGERS)
SALT (ADDS CHARACTERS TO PROTECT STORED PASSWORDS)
securesite (SSL abilitites)
services (services API)
SPAM (SPAM CONTENT DETECTION)
STRINGOVERRIDES (SET NEW NAMES FOR ANY WORD/STRING - LIKE GROUPS TO CULTURES)
subdomain (enabeles subdomain urls for stuff)
table_altrow (makes all tables in content nice looking)
TABBED_BLOCK (MAKES MULTI-TABBED BLOCK WITH NICE ANIMATIONS)
TAXONOMY_MANAGER (BETTER TAXINOMY MANAGEMENT)
taxonomy_other (add terms right from content)
taxonomy_role (set roles that can see terms on add content)
taxonomy_vtn (index/glossary system of all tags/categories)
teleport (hotkey popup nav/search like Quicksilver or GnomeDo)
TEXT_RESIZE (FRONT END BUTTONZ: INCREASE OR DECREAS FONT SIZE)
TOKEN (BETTER HOOK SYSTEM)
TRANSLITERATION (CHANGES INPUT DATA TO UTF8 FOR CONSISTANCY)
USER_BADGES (BADGES/REWARDS FOR USERS)
USERPOINTS (POINTS SYSTEM)
userpoints_top_contributors (lists of top contrib, can views do this?)
userpoints_votingapi (points from voting, rules can also do this)
userpoints_nc (points from content, rules can also do this)
USER_RELATIONSHIPS (RELATIONSHIPS SYSTEM)
userref (attach users to content)
viewfield (attach a view to content)
VIEWS (DYNAMIC QUERY AND DATA DISPLAY)
views_attach
views_block
views_bonus
views_bulk_operations (create pages with filters, like content page)
views_calc
views_cloud
views_cycle
views_daterange
views_gallery
views_popup
views_rotator
views_showcase (broke?)
views_slideshow
views_tabs
VOTINGAPI (VOTING/REVIEW SYSTEM API)
webform (make your own forms and reporting for things like surveys)
wordfilter (censor words, permissions based)
WYSIWYG (GIVES A DOCUMENT EDITOR OF YOUR CHOICE, PREFER TINYMCE OR FCKEDITOR)
WYSIWYG_SPELLCHECKER (SPELL CHECKER FOR USER INPUTS)

It's pretty simple-- whether or not you need to learn anything at all will be directly related to your openness to adjust your vision to existing functionality. Drupal can create all sorts of powerful sites (including full ecommerce ones) without writing a single line of code. That's a fact.

However, if you're a control freak and simply must have your site behave exactly like your vision then you'll need to dive in and get your hands dirty. There's very little chance your imagination will exactly match the functionality of available modules-- my experience has been that it often comes quite close though. If you can live with that difference then you'll be a happy camper-- there's thousands of modules readily available (all free as in beer btw as well). If you can't, don't have the skills to close the gap, and aren't willing to pay for someone who does, then you'll likely be very frustrated and unhappy.

All too many times I see people in the forums struggling violently against their skill level over what amounts to minutiae -- then blame drupal and the developers for not providing exactly what they imagine (which is the way it should should be for everyone afterall don't ya know), lol.