Soul Net's ability is ": Gain 1 life. Use this ability only when a creature is put into any graveyard from play and only once for each such creature." This ability is triggered, with the triggering event being a creature put into any graveyard from play. When a creature is put into any graveyard, the ability triggers, and you may pay to play the ability. If you decide not to play it, you can't change your mind later on.