Across 5 funding rounds, Crunchbase lists Loopt as having raised $39 million and then was acquired (acqui-hired?) for $43 million. He didn't create any multiples of value for his investors. Loopt wasn't a breakout hit like so many other YC startups have been. It was certainly one of the first interesting location-based apps in the App Store, but soon was surrounded by other location-based apps and never really appeared to surface and gain traction.
Obviously Sam runs YC now and has dramatically improved it, but in the lens of being an entrepreneur, isn't he still essentially unproven, and not a success story in the startup world?
But, of course: if the model they're building with YC works, this is the outcome you'd expect. YC wouldn't be very interesting if its outcomes were capped to Viaweb's and Loopt's.
Paul Graham was a borderline founder of Reddit, as well.
That's a successful company by any standard.
I think you're conflating "impressive" and "successful." I would say that based on the numbers you've given, Reddit is an impressive company (it's hard to get any company to that level of private investment and internet fame!), but it's not necessarily a successful company yet.
Being that Reddit's valuation is solely attributable to private investment and not revenue generation (so far), the only sound way to judge whether or not it is a financially successful company is based on its potential return to investors.
By the standards of most VCs in tech, a $500M exit is suboptimal, which will result in either a modest return or just getting their money back. This is not good for a firm's "batting average" when they're trying to beat the SPY with a ten year fund. They need to cover their losses (which, indutry-wide, represents approximately all of their investments), their staff and business continuity costs, their management and performance fees and still beat the market. Modest returns aren't helpful in that regard.
You also have to consider the likelihood that Reddit will transition into a mature business that can exist profitably on its own, without continually receiving significant infusions of cash. If it can't find a way to navigate out of "tech adolescence" it's not really a successful company. The last numbers I read indicated that Reddit has achieved an annual revenue of $8-10M, which is an annual return of >= 2%. Considering that the S&P's average annual return is about 7%, you could earn $10M on $500M by sticking it into an index fund and literally burning the other $25M you would have generated by doing nothing.