I'm going to tackle this question in two parts, starting with how I felt the first time I joined a pro-ana website.
When I first joined a pro-ana website, it was out of curiosity. At the time I didn't realise how easy it was to get sucked into a rabbit hole of being made to feel as though your appearance determines your worth. I was already uncomfortable with my body-image, but didn't join the website (which for the purposes of this post, I'm going to refer to as 'PT') with an intention to join in with the weight-loss 'activities' being promoted on the site. Regardless, I managed to get drawn in by it all, and before long a lot of my diet consisted of lettuce, cucumber, and diet coke. This continued for a couple of months, and I don't doubt that if it had continued much longer I'd have become seriously ill due to malnutrition.
Then there were the photos. There were some that made me uncomfortable, simply because on some level I knew that the protruding ribs and hip bones were not characteristics of a healthy human being. Yet there were other photos that I looked at and admired. I would spend hours looking at photos of girls with flat bellies in a folder called 'thinspiration', or 'thinspo' for short.
Today though, I know I was looking at pictures of girls (and boys) who were dying. I don't know what happened to any of them, I just know that even thinking about the path they were on is distressing for me. Yet this evening I did a quick Google search. Happily, I can say that 'PT' is no longer active. Sadly, though, it's still there, and is a bit of a shrine to everything that happened. I am glad that it's not in use though.
However, a new website has appeared. 'PTAgain' is now active. I quickly made an account to see if anything had changed. It's an awful lot quieter than the original website ever was, with some forums having been inactive for months at a time. I've also not seen a single image of a skeletal person, and I was relieved to see that there was no 'thinspo' folder.
That said, it's not all happiness and light. Within 10 minutes I found myself reading a post from a girl who had eaten a takeaway after an intense session with a psychiatrist, only to go home and overdose on laxatives.
With all that in mind, when I look at the site now, I'm still very much of the opinion that it's a dangerous place, and I will be deactivating my account as soon as possible.
EDIT: Upon looking at the site more thoroughly, there are indeed still photos of skeletal figures and 'thinspiration' folders. Despite a "no nudity or underwear shots" rule, photos of scantily clad girls displaying protruding spines, ribs, hips, and collarbones are unfortunately not hard to find. For a site that claims to exist to support people both in recovery and otherwise, as far as I can tell, the only thing PTAgain will ever serve as is a dangerous trigger for anyone suffering from an eating disorder.