I've been a main of Riven for 5 years, with a 50%+ win rate across multiple accounts each season on Riven. Last season I had a 58% win rate in approximately 200 games with Riven on one account alone.
I have a 20% win rate on Riven right now... what does that tell you?
Riven is currently the weakest she has ever been in the history of this game.
Damage has been increased for every champion due to the new runes almost all having damage tied to them... This makes Riven's snowball not as effective because her shield doesn't soak enough damage even with a lead (and Riven has a pretty massive shield, that's how bad it is).
None of the new Runes are optimal or effective on Riven. She's a skirmisher who must engage often to poke her targets down, which means she doesn't get good usage from Electrocute due to its long cooldown. Arcane Comet gets more CDR from single target abilities, Riven has none. Riven doesn't build attack speed to use ANYTHING in the Precision tree. Riven doesn't build tank stats, and gets minimum use out of anything in the Resolve tree.
Because of the above things, most players are now running the Unsealed Spell book or Aftershock... which both are sub-par on Riven due to their lack of damage. Let me tell you, sub-par runes doesn't stop someone who knows how to play the game. The odds are if you are facing a person playing Riven, it's because they main her. Riven doesn't counter any meta top laners, she's not a meta pick herself, and she is not doing well... No one is picking Riven right now simply because they think she's "OP". If they picked Riven it's because they main Riven and they are confident they're good enough to beat you.