One site, MODS, covered the hunt for the Boston bombers play by play. When the suspects robbed the 7-11, MOD was giving a very acurate description of what was going on, right now to the details of the robbers. We knew not to use cell phones for fear of it detonating a bomb. We knew that a back door to one house was open and a gun magazine was found inside. We also knew one of the suspects was shot and had a IED on him.
Using crowd sourcing methods like those forums and social media is a great way for law enforcement to comb through a large amount of information very quickly. But without the fact checking that goes with the media, it is very dangerous to have them too deeply engrossed in investigations. There have been many occasions where the wrong person has been publically shamed or attacked. And while their hearts might be in the right place, the general public can't admonish them in the same way as the media if they "get it wrong."
Journalists are NOT suspect "hunters!" The majority of journalists "report" the news when it happens to keep the public aware of what is going on. A very small amount of reporters are actually "investigative" reporters. Those who are do not go out looking for "bombing suspects," but instead look for answers to questions in stories. Journalists are not police officers, though.
There is no accountability or fact checking on websites like Reddit or 4chan. Anonymous users can post anything, and in times of stress users can run away with stories that turn out to be false. A good example of this is the report that the Boston Bomber was caught at the marathon, a report from social media that was picked up by mainstream journalists before people realized it was untrue.