I think it would be interesting to list what are the invite privileges of various well-known trackers to donors.
In other words how many invites are offered to donors as a thank you for their donation.
Is there a correlation between the amount donated and the number of invites given?
When the invites are replenished if they are used?
As a start I would say for example that oink gives you 2invites for an initial donation of 5euros or more.
I doubt it that you get another 2invites for a 2nd donation.
I am waiting for your own experiences!