I've come across five 'real life' cases of suicide, two involving relatives, two involving work colleagues, and one other. In each case, there was no warning, the people concerned simply did what they did effectively, and for whatever reason. I'm pretty sure that if I had a serious intention of committing suicide, I would not announce it on the internet or anywhere else (what business would it be of anyone else?), and that I'd carry it out 'successfully'.
I've formed the impression that when people announce in advance an intention to kill themselves, the results are often mystifying and sometimes farcical. I think that some of the people concerned are manipulative fantasists, who may also be dangerous.
Nonetheless, it should also be taken into account that this second category probably includes a much higher proportion of younger people, who may be genuinely disturbed or depressed, and simply have no clear idea what they are doing. On balance, it's probably best initially not to be too cynical, to reserve judgement and to be supportive if one is able to be.