Take the FB offer if you want it. There is nothing wrong with doing so. The startup you are at should be fairly far along given it is at 400 people, but face it even with FB on your resume you'll become more attractive to other startups later. Take the cash, save some money up.

Given you have only been at the YC company 2 months you stand little to no chance of negotiating a higher wage immediately, so even if they do entertain it I'd say they would make it a future agreement. e.g. you do x, y and z then they will increase you to a new salary etc.

In general, don't go to any existing employer with another offer and say I am leaving unless. That is a bad move and makes you look bad and leaves a sour taste with people about you. Instead if you like your current employer and want to stay, go to them and say hey, I feel I am worth more and try and negotiate the increase. If they say no, then explain you understand but feel differently. Tell them it may change whether you can stay at that point because you have X goal, but don't say another offer. If they still don't budge then that tells you what they think of you or the companies health. If instead you do it with the job offer as an ultimatum what happens is they may temporarily give you an increase, but now consider you disloyal and will be putting you and every action you take, or day off you take under a microscope and will replace you in short order. If they instead gave you a raise because you negotiated it without putting them over a barrel with another offer then they won't have that same feeling. Also, sometimes when you approach an employer, as I have suggested, they'll ask if you have other offers already, I wouldn't lie, I'd say yes but I am not here trying to back you into a corner, however I have certain goals I want to achieve and needed to see if my opinion on my value would be validated in the market, as they were I was hoping to stay here but be compensated with T....