What does "not safe" actually mean on this particular site? Is it just that people can see your PM's or email address, find out your name etc - or is it more sinister as in they can access other data stored on your computer?
I've had situations quite often where I'll go to add a picture in an album on my page (back when I use to do that) - and you'll get a whole page of other people's photos. That was pretty weird I must admit. Don't remember ever seeing other people's PM's etc.
This site doesn't exactly have a great track record of security or protecting user's information - in fact, they deliberately avoid complying with privacy regulations and have been very slow at adopting technology for securing the site which was requested by members for years.
Based on what I remember, this site has been hacked (allowing hackers to potentially obtain all information stored), had many glitches (e.g. people would be logged in as other users), and had malware and virus in ads.
For the first two, only information you have shared with the website could be leaked, for the ads, your local phone or computer could potentially be compromised, depending on what the malicious ads targetted and the anti-virus software you're running.
I don't want it to sound like this site is much worse than others - I'm not sure they are. Web security, and especially forums, has been lacking for a long time. But this site is
own by a large company now and isn't just a small pet project by an individual, so that does seem to put larger responsibility on them to make sure the site is as secure as it can be.