I'd like to offer users the ability to post images / links to articles from my web application to Facebook, Twitter, Buzz and any other social network.
A perfect example of the functionality I'm trying to replicate is mashable.com ... where each social network is represented by an icon that a) shows the number of shares AND b) allows users to click on it to post to that specific network.
Don't know if it matters ... but the site is built using RoR.
Thanks
I would first study the available APIs/Webservice documentations of Facebook, Twitter etc. You will learn if and how your features could be implemented. After that you have to write clients for each service. It doesn't matter if you are using Rails or another framework, Ruby or another language. It's all about HTTP requests/responses (or SOAP). There's also a good chance that somebody has already written a library, plugin or gem for your tasks.
Here are the links for adding those social media buttons to your site:
Facebook: http://wiki.developers.facebook.com/index.php/Facebook_Share
Twitter: http://help.tweetmeme.com/2009/04/06/tweetmeme-button/