Every "threat" like that, serious or not, has to be treated seriously. For one thing, there's no way to know for sure. They can't say "Oh, it's just some kid pissed about something" and ignore it. Another reason is that if the person really tries to kill the president and they knew he posted about it on Facebook (or Twitter or whatever) but didn't do anything, they're gonna look like total idiots. And honestly, they don't want that.