I was referring to 2017. Something more like this, but before the start date of Feb. 2018.
@penguinpablo/weekly-report-how-much-steem-is-stored-on-the-exchanges-june-22-2018

Still, 52M at that time on exchange was a lot too.

Like @acidyo said in the comments section, that was 63% of liquid STEEM at the time, so the total figure is not much different from then.
@acidyo/re-penguinpablo-weekly-report-how-much-steem-is-stored-on-the-exchanges-june-22-2018-20180622t133229281z

I think the relevant figures to compare would be HP to SP to arrive at a real liquidity figure - on exchange doesn't necessarily reflect total liquidity.

Here's something from a couple of years ago from @themarkymark that would have left us with around 100M of liquid STEEM at that time. Seems pretty constant.
@themarkymark/how-much-sp-in-tied-up-in-bid-bots

Still looking for 2017 figures.