How much of this is a fundamental behaviour?

Not people moving from Twitter because Twitter is bad and Snapchat is good, but because Twitter is old and Snapchat is new?

Same drive to try newer programming languages and frameworks, same drive to join new startup companies, same drive to throw out and start afresh in lots of domains, same drive to leave this bar and try another.

I see this presented along with "online communities die when they can't block trolls", and "eternal September", and "small communities are more close-knit".

But maybe once it gets old, large, established, you have habits of interaction, same people you talk to, an established persona to keep up, a post history, it gets stale and it feels like work.

If you're a service, you can keep customers long term by offering good service. If you're a brand, you can keep customers long term by staying out of their way and not messing up and giving them reason to reconsider.

But if you're a meeting place, if you're a look and a feeling, if you're a venue for nothing in particular - well, fashions change and online, anywhere else is just as convenient to go to.

Some of it is fashion, some of it is 'eternal september' -- but not all. Frankly, for a lot of Twitter's usecases, other services came along that met that usecase better.

First, Facebook introduced statuses. Most people's Facebook friends were people they knew from real life, so for announcing stuff to people you know from real life, Facebook was where the network was. Later, Facebook allowed each post to be toggled as fully-public, friends-of-friends, or friends-only (with 'custom' afterwards), allowing you to tailor each post to a particular audience. Twitter posts are still always-public or always-private, depending on how you have your profile set. When Facebook introduced IM, the real-life friends circle was forever lost to Facebook, leaving only weaker, or strong-but-online-only, or professional connections to Twitter.

Then Tumblr surfaced, being a snazzier LiveJournal with reblogs. The reblogs serve as both social signalling and icebreaking, to show people things you like or find interesting, to fill out the parts where your page would be otherwise empty or too heavy (full of personal posts) -- it's basically amusing smalltalk. Tumblr was much better at this than Twitter and provided better discoverability.

Then Instagram arrived with the emphasis on a picture, with the social expectation that it's self-made, somewhat curated, something you won't regret later. When Snapchat showed up, it provided an 'unfiltered' alternative to Instagram.

So between Facebook, Tumblr, Instagram, and Snapchat, there isn't much room for Twitter anymore. For those using these four other services, Twitter is probably the place with That One Friend Who Uses Twitter, or a place where you go to try to be witty and clever, but no one reads your tweets anyway. All your friends are elsewhere because the others are objectively better at a particular aspect of what Twitter was once used for, and so are VIPs and celebrities at this point, who've built up presences on every single platform, so no one is missing too much. The only people on Twitter I could possibly care about are tech bloggers, journalists, actual news outlets, or similar, and I don't need to be logged in to consume that content.

>But I think people moved onto other platforms and most (of the cool people) left.

Moved onto what platforms? Left where?

They moved the conversation with their closest connections to Instagram/Snapchat. (Which also allow you to follow celebrities and other popular people) Also a lot of group conversations went to Whatsapp and similar software