In a medium bowl, whisk 2 tablespoons of the lemon juice with the olive oil, 1 teaspoon of salt and 1/4 teaspoon of pepper.
In a medium saucepan of boiling salted water, cook the shrimp until opaque, about 3 minutes. Drain and refresh under cold water. Drain well and pat dry, then halve lengthwise.
Peel the grapefruits; remove all the bitter white pith. Working over a bowl, cut in between the membranes to release the sections. Drain the juice; reserve for another use. Add the shrimp, avocado, mango and herbs to the sections; toss gently with 1/4 cup of the dressing and the remaining lemon juice. Season with salt and pepper. Toss the mesclun with the remaining dressing and mound on plates. Spoon the shrimp salad on top and serve.