Facebook and Twitter didn't grow to millions of users by requiring an initial investment.

Steem is totally different ecosystem. First of all, this is a blockchain and there is a cost for creating a new account. FB and Twitter can create as much accounts as they like, basically without any cost. We can't.

I think requiring an up-front investment would kill growth.

Just look at how much money people are throwing in the blockchain world. A Steem account is valuable cryptoproperty, it's not only a login name to a social media site. We should let our users know this, rather than acting like a Steem account doesn't have any value at all.

And of course I understand how important it is to have way to get accounts for those who, for some reason, don't want to or can't buy an account. In an earlier post I proposed that SBD should be replaced with Steem account token (one SAT gives a right to create one account) and users could earn them as rewards. That would make the system more viral and decentralized.

The value of connected systems often comes from the network effect and who can get it faster.

Read the OP again. ;-)

I know very well the network effect. But it's too simplistic to think that only thing that matters is the number of users. It can turn to a negative thing, too. That leads to chaos.