The internet is huge dump ground full of knowledge and knowhow sharing. A market and meeting place. Given the trillions of websites one must be selective where you spend your time. Certainly StackOverflow is good investement (both to query and to answer).
You ever noticed that you pretty much dont see any jam postings on StackOverflow ?
I also joined LinkedIn (already a few years back) and I still dont understand how many so-called professional groups get flooded with rubbish postings, usually offering jobs where you earn a bomb by filling out surveys and other nonsense. Not sure why either LinkedIn is not capable to sort this out or the group owners let anyone in even without any profile.
For the fun of it (internet forensic for starters)I did a little background check on one of this postings. Quite often posted by someone without public profile, always a woman with a attractive looking profile picture and some fancy names (Cindy H., Evelyn P.,..). The URL in the posting are usually dating sites or other drive surf-by virus sites. You can backtrack an image and check with TinEye where the image is used in the web. I did it with a person called Jessica P. and put the image link into TinEye. It leads to Ukrainian Dating Site. Her name is Irina from Yalta and she is interested in dancing, swimming, shaping, aerobics, travelling. What would she do in an IT forum ? Supposedly she work as a translator. I suspect even this is fake. Anyway you can do the same with these 2..3 simple steps.