It really depends on personality of the trader and trading fees. Personally, I keep my stops very small and its working.
Diversification as pertaining to stocks.... It can be a type of dilution diversification which is counterproductive, however if with it you only target the best quality stocks on offer then agreed, it lowers volatility of returns, reduces stress, the totoise as it were, will outrun the hare.
If you bought FB at the IPO price ($38), I don't know what kind of wide stop would have prevented you from getting out at $20. If you are in it long-term, I believe you should take a proper size (as % of your account) and forget about stop loss. Your exit should be more fundamentally driven than automatic stops.