several factor come in to play with this question
1 is your site public and open to everyone?
2, is it advertised?
3, Are you sure your null is clean i.e either a GYSN or vbteam release if not i wouldn't use it.
4, have you pissed off any members or had any power struggles with past staff.
the final one is normally how jelsoft find nulled scripts followed by leaky nulls after that