They could. They should get to $1-$2 billion in sales and print a Facebook-like margin at 30% net income margins by keeping operations as slim as possible. Decent growth, 40 PE with a public listing (for however long this market stays up), $300m in net income on $1.2 billion in sales, $12 billion market cap. That'd be a real, sustainable business.

That's what Twitter should have done as well. Pinterest won't do it, for the same exact reason Twitter didn't, making the same business mistake with predictable consequences. They all want to be the next goliath and none of them are going to be (and when that reality sets in, they all become reactionaries under pressure from a dozen angles, working from a position of weakness).

Yes, compare this response to Craigslist. The latter is downright successful, dominating and print money for everyone involved.

what do you think is the reason that twitter/pinterest didn't/won't run a lean operation, and why did they choose that path?

There are two types of growth, one that should irk, and the other that should not:

1) Growing for the sake of growth - buying users, anti-user lock-in features, dark patterns, etc.

2) Growing as technology improves - the cost of creating/publishing videos has decline massively in the last few years for the average user, thus products that utilize video to create better experiences should be built.

Because they took a lot of VC money.