I stay away from popular, overpriced tech stocks. I've been burned, badly, before ...
However, in the case of common stock trades, gains and losses are realized immediately. You cannot sell a stock unless someone else is willing to buy it. If no one wants to buy it, you continue to lower the price until someone thinks the stock you're trying to sell is priced correctly (in their opinion).
After the sale, you have lost the value of the stocks in your portfolio, and gained the value of the price paid by the buyer. In a falling market, you will have realized a loss.
If you're the buyer, you will have paid out for a stock, and the current value of that stock adds to the value of your portfolio. You can't realize any gains (or losses) for that trade until you sell the stock.
However, the value of your portfolio is real equity. You can borrow on that equity as if it were real cash.