I also think the site itself is beautiful, but I feel that I need to provide some constructive criticism.
* On the main page, there are links to for Twitter and Facebook, each link to their respective PHP page in the under beta/. The Facebook link works exactly as I would have expected (redirects me to a page at facebook.com). The Twitter link (infogr.am/beta/twitter.php) redirects me to "infogr.am/beta". I was expecting it to redirect me to a page on Twitter that allows me to authorize your application to access my data. I have to click a second link for that. All in all, I think this is just a small bug.
* With text like "it's super simple, just try it!" I really was expecting to "just be able to try it", but when I clicked the "Start Now!" button I was greeted with the need to sign in using either Facebook or Twitter. Having to log in is not the same as "starting now". Definitely not what I was expecting.
My recommendation is to change the wording so that it is completely understood that to even try the service I need to log in with my Twitter or Facebook account, or allow me to just use the service, and when I wish to save something, prompt me to login. Personally, I recommend the later.
* Concerning the use of social media sites as login providers. There was no mention in the "Making infographics is very simple!" section that I can use my Twitter or Facebook data to create these infographics so why does Infogr.am need access to who I follow and my tweets?
Now if you are using Twitter and Facebook as login provides because its easy, I completely understand. Then do the user a favor, on the "/beta" page, let me know right up front what you are going to access, what you are not going to access, and why you need anything you are accessing. I say this because clicking on Facebook login button redirects me to a Facebook login page that doesn't even tell me what you want to access.
If you want access to my data, be completely up front and honest with me. Twitter does, but do I really need to click through two pages to find this information out?
All in all, I really like the idea, and look forward to trying it, but I am going to wait till I don't have to use my Twitter or Facebook account to log in.