Help me with [function.fsockopen] issue...

Hi, hoping someone can help. The following error occurs on my WPMU site pretty much all over. It's now starting to appear inside the widgets on the widget layout page.

Warning: fsockopen() [function.fsockopen]: unable to connect to twitter.com:80 in /home/cheeks/public_html/wp-includes/class-snoopy.php on line 1142

I can obviously determine that this is a failure to open a connection to Twitter on port 80, but I have two questions:
1 - what plugin is this originating from? So I can disable it and work around the issue
2 - why is it failing? Is it something I have to fix up on the server? or is there somewhere in WPMU that needs configuring? i.e. how can I fix it?

Thanks in advance,
Paul