I've been flying for more than twenty years, and have spent a lot of that time talking to ATC as they tracked me on radar. Here are my observations, combined with some technical background learned during training:
Within or near any large city (which will typically have multiple controlled airports) there's virtually no place that is below/outside radar's range. You are typically being interrogated by multiple radar units, and at any given moment at least one can track you unless the city is in very mountainous terrain and you're in just the right spot.
Outside of populated areas (or more accurately, in areas without large controlled airports), it is quite easy to be out of range of radar, either by flying low (which BTW is no challenge at all for a 777, it's simply slower and uses more fuel), or more likely, by not following established routes. Keep in mind, in addition to having distance limits, radar units tend to be tilted upward a few degrees because they're optimized for maximum range in the area they're needed - and that's at altitude, along established routes. Any coverage off those routes is mostly by accident. There can also be dead spots directly over the top of a radar dish in some cases. I've flown distances of up to about 50 miles while talking to ATC as they had no radar contact at all - and it usually happened by accident, while flying at least several thousand feet above the ground. I'm pretty sure I could fly across an entire state without coverage it it were a goal.
That said, a terrorist or any other hijacker is unlikely to know how to avoid radar with complete success. They'd have to know the quirks of radar coverage in the specific area, and be in absolute control of the plane because any airline pilot would be smart enough to stay in range even under duress in a hijack situation.
I feel sure this plane was not hijacked. It disintegrated in flight, whether due to an explosion on board, or because of a mechanical issue which caused it to endure unusual flight attitudes which the airframe couldn't survive. It could have been sudden full travel of a control surface which ripped part of the tail or wings off. It could have been hit by a military rocket either by accident or on purpose. It could have had a fuel tank or something in the cargo area explode by accident. And yes it could have been a terrorist on board, but I think that least likely because they usually can't wait to take credit for such events.