You can't really point to other images and say, "see what they did". All images are different and in many cases (with professional images) the shadows are part of the design from the beginning. This one it's obvious it's not intentional given the location and pose. It is distracting to me as it was the first thing I noticed. I looked right at her crotch because of the dark line I saw silhouetting it. This coupled with the low angle looking up at her really kind of detracts from the image IMHO.
But we've probably beaten this to death.

If you're happy with the results that's ultimately all that matters. I've had people offer opinions on some of my images I didn't agree with and even if I had it to shoot over again I would have done the same thing.
We have beaten it to death, but here's another stick for that poor horse

All kidding aside, I never put the images up for critique, but just to show something about the Fong, but that shadow thing started to make me wonder if something was wrong on my end. That's why I really wanted to know what everyone was seeing. Beyond that, it's really just opinion and nothing more.
Nothing personal to anyone that has responded, but the only opinions I tend to care about come from paying clients, editors, family and close friends, and photographers whose work I truly admire - and that admiration is built up over years, not a few posts. It's not that I can't take criticism, and if I think someone has a point I'll put some thought into it, but if I'm going to put something up for critique it would be, well, a very different picture. Nonetheless, I appreciate all the replies.
The shadow is not that bad. The pose looks a bit awkward, but the shadows cast by the flash are not 'nasty', nor are they excessively dark. In the beach shot the flash doesn't seem to add too much except perhaps minor fill and a small catchlight; the shot the Fong diffuser does add to is the shot on the porch, where I can see it is adding a very soft, very slight degree of specularity to her skin, which adds a bit of dimension.
Given the limitations of most d-slr cameras and the power of 'most' speedlights, like those in the Nikon SB 800 class or the Canon 580 EX-II class,
with the shutter speed limitations on flash synch at 1/200 or 1/250 and base ISO settings of 100 to 200, with most cameras, the light from a Fong diffuser is really only going to provide slight fill light, eye catchlight, or slight amounts of specularity most noticeable on darker-skinned people or those who are sweaty or wet...
I mean, the Fong in the first picture is not providing much of the exposure, so the shadow is not all that bad....it's clear that even in a shaded area, the flash shadow is so not-dense that the Fong is just a very slight, weak fill light--but it *is* adding to the ambient light. In fact, the one good thing about the FIng is that its huge loss of Guide Number means a shot with it brings the flash down to a nice Minus 2 to Minus 2.7 f/stops, which is almost exactly where you want to be to get almost-unnoticeable fill flash in sunlight. In the shade, it's a bit more-noticeable, as seen on the side view shot of the girl on the porch; there the flash effect is more clear,and I think more dramatic.
I know what you're saying--none of this are killer, portfolio shots, but they do show me what the Fong diffuser looks like in three types of shots.
Good that you get it too. I bet with a less powerful flash, the Fong would probably just about kill the effect of the strobe in the sunlight. A similar effect, outdoors, can easily be had by stopping down the flash right around where you mentioned.
Alright, let's ship this poor dead horse out to the glue factory. Now I have to go hunting for a good wide-angle for the Canon A-1, so I'm sure I'll have some questions on FD lenses soon enough