So Riven got a minor buff this patch which added some damage to her Ultimate active. This was a rather.. Controversial change, to say the least. The question now is, what happened?
Let's take a look at the stats. Rivens winrate dropped from 50.74% avg to only 49.85% which is to be expected as her playrate went from 9.68% to 15.38%. So you'd expect to see a significant drop in avg. games played, right? Shouldn't a higher playrate be explained by newbies trying Riven due to the buff? No.
Her avg. games played increased from 14x (iirc) to whopping 174 games. Only 6.93% are new to Riven (1-5 games played) and 37%+ have more than 125 games on her (duh, her avg games is 174..).
This might irritate some that are not toplane mains. The thing with toplane is that basically every toplaner has at one point of hit toplane career mained Riven or atleast played her alot. Riven is _just that much fun_. Especially us toplaners who are used to playing boring TP tanks who have 0 impact before the teamfight phase starts just LOVE to have a champion that is able to roam basically from lvl 6 on. A champion that allows us to play toplane like it was in season 3, with Ignite in our left hand and ham instead of brain.
So those people are now returning to playing Riven. It's useless to pretend that Riven is an easy champ, she has never been. Newbies have a <43% winrate. Riot was always aware of how popular Riven is and that it is necessary to keep her sort of weak because otherwise she'd be P/B.
Conclusion: this doesn't say anything about how strong Riven is compared to the last patch. Just keep one thing in mind: the average Riven you meet has MANY Riven games. She knows the matchup better than you in 99% of the games and she has a lot of fun playing Riven. Not because she is actually winning many games (look at the stats, it's neither easy nor especially rewarding to play Riven) but just because she enjoys the gameplay.
So when Riven is deemed "OP" she is usually (S4 is an exception) not OP in the sense that she is too strong for the amount people put into her. She is too strong for how popular she is.