It depends on where you want to put this link.
If it is in a UITextView, you just have to enable it.
textView.text = @"Some text with link in it : http://http://stackoverflow.com";
textView.dataDetectorTypes = UIDataDetectorTypeLink;
More info on iOS reference library.
Or, if you want to open Safari programmatically:
NSURL *url = [ [ NSURL alloc ] initWithString: @"http://stackoverflow.com" ];
[[UIApplication sharedApplication] openURL:url];
[url release];
If you want to share on Facebook, you need to tell Safari to open a URL which will display a Facebook page that allows the user to share. Here is an example:
NSString *urlString = @"http://stackoverflow.com";
//The url you want to share
NSString *title = "The Title of the Page";
//The title you want to be displayed on Facebook
NSString *shareUrlString = [NSString stringWithFormat:@"http://www.facebook.com/sharer.php?u=%@&t=%@", urlString , title];
//Create the URL string which will tell Facebook you want to share that specific page
NSURL *url = [ [ NSURL alloc ] initWithString:shareUrlString ];
//Create the URL object
[[UIApplication sharedApplication] openURL:url];
//Launch Safari with the URL you created
[url release];
//Release the object if you don't need it
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…