Well, on this site - not posting story lines is to avoid messing
Jim up on any of his books.
I would think that when you post on any of these critique sites you do run the risk
of running into someone with a similar story line or, possibly,
in having someone borrow your story line inadvertently.
It happens - Harlan Ellison was at a writer's workshop and
lo and behold, some kid who didn't know Harlan at all came in
with a story very, very similar to Harlan's story - Bleeding Stones.
Harlan made arrangements with the kid because he'd heard the
kid's story. So if it can happen to Harlan it can happen to anyone.