It also depends on the locale the growers is in. Growers in the U.K. are almost undoubtedly going to be left alone, and it's too much effort for Canadian law enforcement to set up operations and bust someone on here. As the other posters have said, they need real tangible evidence that you're growing in order to obtain a warrant. The scales could be tipped against these growers however if they're consistently producing a ton of marijuana, and the law enforcement needs a headline in the papers.
In the U.S., it likely depends on the state. The DEA has a general policy to overlook small time growers. I'm sure that they patrol this site on a regular basis, but they're only going to devote the resources to bigger growers. They might also tip off local law enforcement agencies, but as I said, it depends on the state. If you're only growing a few plants, and live someplace like California or Ohio where marijuana is decriminalized, it's not worth the effort. You'll probably just get a hefty fine, and in the aforementioned states, no jail time. I'm not growing now, but when I have in the past, you can bet I made sure I stayed within the "reasonable" parameters of the law, since it is decriminalized where I live. When law enforcement agencies bust people, it's so they can get some notoriety or have something to show their superiors/the public. It's not necessarily about interrupting the flow of illicit drugs, because it's been consistently shown that even federal enforcers barely make a dent. It's all about public opinion.
But God help you if you grow anything and live in Vermont. If cruel and unusual punishment wasn't outlawed, they'd probably chop off your pruning hand there. Check NORML.org for your state laws. Grow small, and grow only for yourself (don't sell), and no one will likely want to bother you. Trafficking and large harvests are what they're usually looking for. Not small-time recreationial or medical growers. Me, I prefer to stick with growing legal psychadelic plants, for the sake of my own health.
~Ethno
(P.S. Oh, and plenty of people on here have been busted. Though I'm not sure whether it's ever been directly because of this site)