I'm currently building a "social bookmarking" tool, so can't speak to first hand evidence but have looked into this and spoken to people (2nd hand, not 1st hand sources).

[would love to chat over coffee or email with others passionate about this space. @mceoin on twitter]

Why delicious died:
- Yahoo acquisition. Death by a thousand cuts from that day forth. Some things here really pissed off core users include Yahoo sign in and shuttering some core features that power users loved (see exodus to Pinboard).

Landscape:
- The attention economy has definitely shifted. Perhaps delicious could have evolved along with it (we'll never know), but now it is certainly the case that it is more difficult to gain mass adoption for products that will provide you with lists of information, since 'good enough' alternatives exist (notably, these are typically feeds, not lists)
- the world has gone mobile.

Who lives, who dies, who tells their story?
- Most died or were acquired (it turns out if you develop a team of search and indexing experts, you're pretty attractive to big companies). I counted 20+ here, about 50% acquired, the rest: ??
- Pinterest had the visual twist, and grew to 200m people
- Pinboard generated revenue from Yr1, and has a small but profitable user base (important, because "not dying" is a core piece of the puzzle).
- Kippt had a great product but team was ultimately were acquired by Coinbase, so it would be awesome to hear more about their experience.
- Pocket lives on through Mozilla acquisition. In my experience they never cracked the social piece, although definitely provided the individual utility to be a daily-active-utility. Watch-this-space.

Why is this space difficult, generally?
You would think that being able to explore lists of what people find interesting is appealing, and it certainly is (in theory). There's a hierarchy in information discovery for this space where: personal saves > friend saves > followee saves > random people saves.
- social products require high clustering coefficients (very important). This seems to be particularly difficult to obtain in social bookmarking space. (I believe, but cannot confirm, that most products went too wide too early, and failed to nail this experience. Facebook's control of launch from college to college is a different but effective strategy that gave them saturation, so I believe a restricted entry via referral is pragmatic approach here.)
- Low virality. This is a hard one to prove since non-success doesn't mean no potential for breakout success.
- Low target market. Same point as above.

Monetization (three options I see):
- advertising: requires mass scale, hence most attempts here die.
- pay to play: Pinboard is doing this. (does this preclude wider adoption? or are other factors at play?) A balance of paying for privacy features might be an option here.
- crowdfunding/non-profit: e.g. wikipedia model. Nobody has tried this yet. Would take a certain kind of team and community, but might position itself well for being a long term human-curated archive of the web.

I have some thoughts on how what this space lacks and what can be done differently that I'm happy to share privately. Suffice to say that we want this, and believe there's an opportunity to build a product that at least a few people love.

Who is taking a crack at at "social bookmarking" today?
- Refind.com. Notably, they're more "show me what my friends are reading right now" and feel like a slimmer, more reading focused, twitter feed. Mobile app is best consumption vehicle.
- Mix.com (from Garret Camp/Expa). Hasn't 'clicked' for me personally, but if might for you.
- Us! We're not ready for a Show HN but if you're in SF and love this space, please do hit me up on twitter for a coffee: @mceoin.