I've spent many hours on bodybuilding.com and in the forums, reading, researching, etc.
Many of those people have no idea what they're doing, and just cause fights, bicker, debate over nothingness.
That being said, there's lots of good info in there too if you sift through most of the junk.
Many of the people on bodybuilding.com advocate the whole bulk/cut routine, which is (I think) generally a good idea.
But many of them do it sloppy, and they do technically "bulk" but they also put on much more bodyfat than they needed to.
Then they end up losing a lot of their gains by cutting too hard.
I know I've been there before.
And many of the people on bodybuilding.com take sketchy supplements as well.
It isn't always the best place to get good advice unless you know what to filter out.
How did I gain 15 pounds?
Just by eating about 500 calories over my maintainance calories and keeping my protein generally high (somewhere around 1 gram per pound of lean weight.)