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That's assuming the tweet is only text. I had used it to quickly bookmark links within tweets, but opening them and saving them to Instapaper honestly works better.

They were testing as far back as June, but probably even earlier: http://mashable.com/2015/06/17/twitter-favorite-hearts-stars...

I actually come back to a lot of saved Reddit posts, particularly ones on programming subreddits where I can't easily read the article or mess with the code on my phone.

That's the point - a Reddit post can be much more substantial than a tweet usually is, so saving makes more sense for the former than the latter.