In your CSS, change #rightdiv to a class,
.rightdiv, so you can re-use it. See class vs div here.
Then, to get floats to work, they need to be in the right order. Move your #leftdiv up so it comes first and make these extra changes highlightedin red -
" height="60" scrolling="no" width="468"></iframe></ins></ins></p>
<p><em>Welcome to venTime! Consider this your venting spot where you can come to let it all out. Is there no one to talk to, the boss creeping by, or a gripe burning a hole in your back pocket? Well, don't stress anymore - we're here to listen! So take a deep breath, let it out, and get on with your day. At venTime, the grass is always greener!</em></p>
<h2>try it out: </h2>
<p></p><div class="fb-comments fb_iframe_widget" data-href="www.venTime.com" data-num-posts="8" data-width="380"><span><iframe src="https://www.facebook.com/plugins/comments.php?channel_url=https%3A%2F%2Fs-static.ak.fbcdn.net%2Fconnect%2Fxd_proxy.php%3Fversion%3D3%23cb%3Df2a91f0f507f17c%26origin%3Dhttp%253A%252F%252Fwww.ventime.com%252Ff3b1779e6976ce2%26relation%3Dparent.parent%26transport%3Dpostmessage&href=www.venTime.com&locale=en_US&numposts=8&sdk=joey&width=380" class="fb_ltr" style="border: medium none; overflow: hidden; height: 1912px; width: 380px;" name="f2c00ef2337c96" id="f6d408c4866f18" scrolling="no"></iframe></span></div><p></p>
<h2>how it works</h2>
<p><em>To air your grievances, simply click on the “Add a comment” button. If you have already logged into Facebook, you’ll find that your profile picture is to the left of the comment box and you’re ready to post. If you have not already logged in, you will be asked to do so once you click the “Comment using…” button. Simply click on Facebook and it will bring you to a login page. Unfortunately, the Facebook comment box does not currently allow users to log in through the other options listed. We’re looking into it!</em></p>
<p><em>Additionally, vents may be sorted chronologically as well as by the user’s social rank. The drop down menu to sort comments may be found directly beneath the words “try it out” to the left of this box: just click on the small blue downwards facing arrow. Reverse chronological will sort vents so that the most recent ones appear at the top, while chronological will start with the very first vents.</em> </p>
Now you've made another div here.