There are several of these petition websites and I’ve signed for a few things that I felt strongly about. You receive updates on the going petition, and they often do seem to be fairly serious and have some effect. Of course, in the end it’s still just a petition, not a magic wand.
It is a sort of slactivism, absolutely.
I think you can choose what sort of spam you want to receive from these websites, I don’t receive any so I have no problem. I probably just selected the boxes for “no updates”, then you can select “updates” for the particular petition you’re signing. They also often ask for an address (presumably so you can’t enter 20x) and I have also never encountered a problem with that.
I just signed one by change.org asking not to send the Dutch King & PM to Sochi. It was organised by the COC and, I am told, promoted on that tv show with Albert Verlinde, so I presume it is somewhat serious. It was reported in the NRC for getting 20.000 signatures, so it can’t be going by unnoticed.
ETA: this is not to say that things advocated by Albert Verlinde are in any way to be thought of as “serious”, but just to say that there might be a wide audience and notice might be taken…