I know exactly what you mean...falling behind top, ESPECIALLY IN THE FIRST FEW LEVELS, makes it super hard to have good trades, and if the opponent is any good, they will push that advantage hard. At that point, you have to simply out play them or get help from your jungler.
In that situation as Riven when you expect a gank, I'd say stay on the side of double bushes top as much as possible and save your CDs for escaping rather than any harass. Riven has some of the best escape abilities, including an AOE stun...so if you stay on the side of the upper two bushes, you have enough time to make a pretty easy escape.
This is augmented by the fact that Wu and Xin will try to use their gap closers...as soon as they use them, just W, then E and Q away. I don't think you really need to deny yourself any farm or exp in that situation.
Here's what happened:
We're blue side.
Riven got a lvl 2 gank from Noct straight from blue and blew Wu's flash. Riven proceeded to get first kill (not fb) in the lane on Wu.
Later, Riven gets ganked by Xin (with or without red, I don't recall) and trades kills when they turret dive.
Then, after some dueling, Wu barely scrapes a kill off of Riven due to a misplay.
Then, Wu gets babysat by Xin with red buff. Wu freezes lane in middle and farms maybe 2-3 waves of CS during this time.
Also important to mention, at some point before the babysitting, Riven, after getting Wriggles and Ninja Tabi, buys a HoG.
I was Soraka with CV during this time and we had vision when Xin would come top and when he started babysitting.
So many mistakes, both item build, dueling and farming that occurred. While I agree that in the last scenario, Riven couldn't venture out to farm or gain experience, it was all the blunders before that cost him the lane. He has no right to complain.
He should have begun dominating the lane after the first kill, while warding his lane. Which he did. But he shouldn't have died from the gank because he saw it coming. He shouldn't have lost the 1v1 duel. He shouldn't spend 835 on a HoG, especially when behind, especially on a champion where the value is health is superseded by AD. Could have easily been DBlades and Pots just to establish lane dominance and destroy Wu if he couldn't farm enough for BF sword.
Riven vs Wu = ez win for Riven.
Zak better see this. Tried to tell him last night, but he was upset after loss.