Definitely it needs activation of platelets for release of granules containing various factors .If you are performing experimental work you need exogenous activation if talking about inside the body endogenously substances are there or factors like endothelial damage etc  

To release granular content platelets have to be activated. There is a lot of literature on various conditions and how to do it gently and how to measure each particular compound. 

 It depends on what you mean by "pre-activation". To release growth factors, platelets do have to become activated. But, if you are asking whether pre-treatment of PrP with an agonist is necessary, the answer is no, as  activation may be spontaneous, e.g., as you treat an implant surface with PrP or place it in the wound. However, I think the outcome will depend on the PrP preparation/activation protocol (including agonist treatment, if any).

Merely the injection of PRP without an exogenous activator such as calcium gluconate results in a release of growth factors based on the response of hair growth following injection of not activated PRP.  We found that both not activated and activated PRP increase hair density, but activated PRP increases density to a greater degree.  The method of activation, however, can influence the concentration of growth factors.  In our ELISA studies, we found no statistical difference between the concentration of growth factors when comparing the concentration of growth factors activated with calcium gluconate and bovine thrombin.  However, we found that the concentration of high molecular weight proteins is much higher (6-8 times) when we lysed the platelets with sonication.  After this study, we found that the higher concentrations of growth factors significantly increased the rate of graft growth following hair transplantation.  We believe that the fibrin clot traps some of the growth factors, particularly the larger proteins in the mesh.