These applications have (at their core) two functions: (1) collect a piece of data and (2) expose it in a useful way (usually with an API).
They tend to focus on atomic data units of a single type (they don't collect just anything).
I'll try to offer some examples since I think that my description may be unclear.
Last.fm (the old site) atom: a song listened to collection: automatic (scrobbled from media player) expose: web app, API
Reading.am https://www.reading.am atom: article that you read collection: manual (post to web app), automatic (hooks to Twitter/FB/etc) expose: web app
Pinboard.in https://pinboard.in atom: url collection: manual (post to web app), automatic (hooks to Twitter/FB/etc) expose: web app, api
A good litmus test might be how cleanly they map to IFTTT or Zapier workflows. Looking for the Xs and Ys in "Add X to Y."
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Others to consider: - Original Twitter? Maybe Twitter's atom was a status update (or a thought) - Original Delicious? Atom was a URL
Early Twitter and Delicious are confusing since they were both data repositories and social networks.