Does anyone know how much to use of this to flavor one 8oz glass of sparkling water, or maybe even a liter bottle (sodastream)? thanks.

I use 1/4 tsp for a small Sodastream bottle or 1/2 tsp for a large bottle (plus equal amounts of Whole Foods liquid stevia). We've liked rum, orange, strawberry, almond, and rootbeer using roughly these amounts. Some flavors are better with a little more sweetener. Hope that helps.

Can this be used in place of triple sec for making margaritas?

Actually, it absolutely CAN be used in lieu of orange liqueur (triple-sec/contreau/gran marnier ) in a margarita!! You obviously wouldn't use the same amount since this is highly concentrated, but I've been using it for a while in margaritas with AMAZING results!! Here's my recipe for a nearly 100% sugar/carb free margarita (under 3g net carbs, from the lime juice): 3oz tequila 1.5oz lime juice 10 drops of orange extract 10 drops stevia sweetener (12 if you like it a little sweeter) 1.5oz to 3oz of water (depending on how strong you like it) This will make a 6oz-7.5oz margarita with the following nutritional info: 200 Calories (190 from the tequila, 10 from lime juice) Carbohydrates 3.1g -Fiber 0.2g -Sugar 0.6g -Net carbs 2.9g Sodium 7.4mg You can also make it frozen by just adding it to the blender with a shaker-full of ice for 2 servings of half-shaker for one.

Is this oily? Oily flavorings will prevent meringues and marshmallows from whipping.

Although it doesn't feel oily to the touch, it's made with orange oil. I looked up recipes for meringue cookies and several use orange or lemon extract. I looked up tips for making meringue and it said if you got a little egg yolk and couldn't fish it all out, it would just take longer to whip into stiff peaks because of the fat in the yolk. When using extracts, though, they are added after the meringue is whipped. They usually only use 1/2 tsp of extract, so I think they would be fine.