Use 3m Exterior molding & trim tape (the grey foam stuff, available at most auto parts stores) it's the same stuff the dealership uses when they mount them, cut a small piece the length of that front tab that's come unglued, clean both surfaces Inside of skirt and on the car's sheetmetal with rubbing alcohol on a cotton ball or soft rag. press the foam tape onto the back of the side skirt, over the existing adhesive tape, and then, peel the backing, and press it onto the car. I've fixed both sideskirts using this method. the new tape will be wider than the stock tape, so just stick it on low enough that the top edge is lined up with the old tape, and you'll be fine.
I suppose i'll need to do this again, as the last bit of factory tape came loose on the rear part of the passenger side skirt, all my repaired areas are sticking firm and have yet to come loose.
BTW, don't use liquid nails, that's just plain retarded! it sticks great, and is very useful, but if down the road you need to replace a side skirt due to damage, you will regret it (liquid nails is a hard product to remove from anything!)