How can we measure whether Ethereum as a technology is "picking up" or slowly losing relevance? - Number of individual accounts? - Number of smart contract? - Overall hashrate?
I think Overall hashrate would be the best measure. To increase hashrate people must actively dedicate hardware and time, and it therefor acts as a rough count of the number of people involved, weighted by how involved they are. In contrast to the other two options you proposed:
Number of individual accounts: This rises as ethereum becomes more popular, but does not decline if it becomes less popular. Also creating an account involves almost no effort past installing; These people have shown some interest but aren't neccesarily dedicated yet
Number of smart contracts: This can be affected by too many things, as one person making a million contracts can certainly skew your results.
All in all, I believe the overall hashrate would be the best objective measure here.