Now you can  run multiple blogs from one installation of WordPress. This new feature is being called Network or Multi-site option in WordPress. All you have to open your wp-config.php file located in your WordPress root folder. Then add the following line anywhere in the file:

define(‘WP_ALLOW_MULTISITE’, true);

Find more detail on ” How to enable Multi Site feature in WordPress 3.0“. After doing this you are able to access the network page from Tools » Network. By default, you are allowe to run other blogs as a subdomain. You can use other domains for blogs by using  a Domain Mapping plugin (Detail: How to do Domain Mapping in WordPress Multiste Network).

After converting your WordPress single installation into Multi Site Network,  you can do some cool things with it. I will see how to get posts from all blogs in the network.

What if you want to get some recent posts from last updated blogs, not only one blog? Then get_last_updated() is the function that will save your life!

See a simple example that shows last post from last updated blogs:

$blogs = get_last_updated();
echo '
<h1>Last posts in network</h1>
';
foreach ($blogs AS $blog) {
	echo "
<h2>".$blog["domain"].$blog["path"]."</h2>
";
	switch_to_blog($blog["blog_id"]);
		$lastposts = get_posts('numberposts=1');
		foreach($lastposts as $post) :
			setup_postdata($post);
			the_title();
		endforeach;
	restore_current_blog();
}

(Source: wordpress-multisite-blogs)

Reference:

You can read more about WordPress Multi Site Network:

Protect Your WordPress Multisite Network From Shutdown With Anti-Splog and Post Monitoring
How to Get More From WordPress With Multisite Blogs (Network Install)
Best BuddyPress Themes for Social Networking
How to manage plugins in WordPress MultiSite Network
WPMU Functions/get last updated
http://codex.wordpress.org/Category:WPMU_Functions
http://codex.wordpress.org/Function_Reference#Multisite_functions

These are WordPress Themes, which you can use for your Multi Site Network, because they support BuddyPress and BBPress.

ReviewIt – Community WordPress Theme

ReviewIt provides you with a powerful review and community theme, that is also 100% BuddyPress compatible!
Key Features

• 2 Unique Skins – Dark and Light
• Custom Post Types – Reviews and Slides
• Powerful Review System – Utilizes GD Star Rating WordPress plugin (requires PHP 5 )
• Easily Change Tags – Switch the site from movies to games, apps or whatever you want
• Multiple Review Displays – Display reviews in lists, by score, date, based on similarity to other reviews etc.
• Total BuddyPress Support – This theme supports BuddyPress but works just fine without it

Live Preview | Detail

DynamiX – Premium WordPress Theme

Theme features:

• Dual Skin Mode
  – Inner (Main body content) & Outer (Header / Footer area) Skin can be selected
  – Select Skins for entire website and per page
  – Select from 20 Outer Skins and 2 Inner Skins
• Non-Post Gallery Slide Manager (Manage all your Gallery Images/Videos in one area)
• BuddyPress Theme (BBPress Support)

Live Preview | More Detail

TheAllure – Elegant Corporate and Portfolio Theme

TheAllure is a professional and very flexible WordPress theme packed with amazing functionality and a very impressive Design. The Theme has Unlimited color choices. It is packed with lots of shortcodes and a powerful admin panel (PowerPanel) that lets you control every aspect of your website.
The Allure theme is very easy to set up no matter if you are a WordPress beginner or a WordPress pro.
It is also compatible with WordPress MU (Multisite/wp_mu). It is WPML Translation ready and for your convenience, already includes 19 languages for the frontend.
Key Features

• Advanced Admin Panel packed with Options with over 20 sections to help you control every aspect of your website (SelectedTheme’s PowerPanel)
• Unlimited Colors
• WPML and Translation Ready
• 19 Languages Available
• Cufon Font Replacement with over 25 fonts
• Custom Post Types: Portfolio Items, Slider Items
• Supports Custom Menus
• 3 Sliders: Nivo Slider, Thumbnail Slider, Full Content Slider
• Fully Flexible Homepage Layout
• 50+ shortcodes
• WordPress Multisite (WordPress_MU) Support

More Detail

Following are the WordPress Plugins which will allow you to share your blog post on Social Networking sites and you can sell your premium content to your blog users:
SocialPress
Sociology for WordPress: Twitter/Facebook Poster
WordPress Premium Content