In fact, the most part of this «challenge» was created by russian media for promotion laws for internet censorship and the most «victims» are fakes or trolls (not all, of course).

A short story about it:

1. 22 Nov. 2015 — Рина Паленкова (Rina Palenkova) published 2 selfies with a railway in a background and with an inscription «ня.пока.» (nya.bye) in Vkontakte (russian facebook). On the next day, she committed suicide — crushed under the train (the reason for this act is unhappy love).
2. Users of Russian imageboards started sharing this situation and this phrase (ня.пока.) became a meme. The result is many russian teenagers started publishing similar selfies and this inscription.
3. And of course, someone did wanting to get money from this situation — the girl was subscribed to the «f57» group and the admins of it started sharing gossips that the reason for the suicide was psychedelic videos from their group. Then several other people created similar groups.
4. The admins of these groups did announce «flashmob of suicides» but the result is all «suicides» were fakes — the most members of this «flashmob» were admins themselves.
5. 16 May 2016 — all these groups had no more than a few hundreds of subscribers, almost all forgot «ня.пока», and the most admins lost interest. But in this day, Novaya Gazeta (a Russian newspaper) published a big «research» about these groups. This «research» has many inconsistencies, mistakes, and the minimum amount of serious evidence. However, it got more than 1.5 millions of views in the first 2 days.
6. Hello a moral panic, real suicides, and new laws for internet censorship.