You can say the wrong thing to your neighbor and get in trouble these days. As far as posting on line I wouldn't post any crime I was involved in actively or anything I was criminally thinking of doing. In this day and age whatever you say on your call phone or in an email is just as culpable as posting on a public forum.
I assume you meant if you admit to doing illegal things on here can law enforcement use that against you? Yes I assume they could in most places, if it was something that was in fact actually illegal, was something they could currently prosecute you for (drug sales, violent crime, etc), if they had conclusive evidence that it was you who made those posts, and they were able to get the posts accepted to be used as evidence. They would need evidence aside from the posts though. Or LE could act on info they got from posts about illegal acts (for example, say you posted that you were doing a drug deal tomorrow and they had enough info to figure out where that was and show up there).
However, please bear in mind we can't really provide legal advice and laws vary greatly from region to region. Please be careful and use common sense when posting on Bluelight.
This thread isn't appropriate for Basic Drug Discussion though (in fact, I'm not sure whether it's suitable for anywhere on the site, but you can let me know if you have questions that aren't covered in the guide or you can contact a lawyer if you are still concerned and want definitive facts on what you should/shouldn't feel safe to post).
I'm going to close this. Please PM me if you have any questions/concerns.