Something tells me the blockchain activity chart is (intentionally? ;o) ) misleading. While pretty much all Steem transactions have monetary impact only the tiny fraction of them are actual money transfers, so if the blockchain activity counts upvotes, resteems and comments it makes no sense to compare it to activity in f.e. BTC. It does say something about Steem - that the DPoS scheme it uses has the capacity to process large amounts of transactions. It says nothing on Steem relative popularity though. In order to get honest comparison we should match it with tweets or Facebook posts and likes. That would tell something on Steem adoption rate. Comparison with tweets/posts/likes per user would tell us about how active are typical Steemians compared to Tweeter/Facebook users.
It's just exactly as advertised. It's not about money transfers nor popularity on social media, it's about transactions on the blockchain.
Adoption is a different story. Obviously. With some exceptions, currently, to get STEEM, you have to get BTC first.