You don't need a big exit when you actually make money. And since they are middle men for cash exchanges I don't think making money will be too much of a problem.

With 50 employees in SF I very much doubt that Patreon is anywhere close to being profitable.

Even if Patreon doesn't have a big exit, it could still make a profit, and that profit could then go back to the investors.

According to the article, this gives Patreon a $47 million valuation, and Patreon has 17,000 artists.

That is only ~$2,800 per current artist.

Given that this investment is made with exception of growth, this does not seem like an unreasonable valuation.

$47M isn't their valuation, it's the amount they've raised. No company would raise $30M at a $47M valuation unless they were very desperate.

The valuation is likely something like $150mm. (20% ownership for the B round post money)