empty json response?

I am learning wireshark while trying to learn how ajax works... using 500px as example, and picking a random portfolio, http://500px.com/Pop315, I see this traffic...

1. browser requests page from 209.15.228.200, gets the page, which essentially contains just javascripts and div place holders.
2. browser requests external javascript and an image from 72.21.91.19, gets both
3. browser sends a "GET /event?a=..." to 50.17.242.30
4. 50.17.242.30 returns a json object consists of just "{}"???
5. browser talks other websites like goggle analytics...
6. now somehow, the browser knows all the thumbnails to get and asks 72.21.91.19 for all the tumbnails to display on the page.

Looks to me, the browser javascript engine figured out what request to send to construct the page in step 3, then got that reply in step. However, the reply was just empty "{}", how can that be? If someone can run a quick test, I would really appreciate it.