if facebook start accepting bitcoins Bitcoins price will go up

All people will do advertise by bitcoins

i doubt. Facebook has begun to dim, too many ads are displayed.
if any facebook accept bitcoin, ads that appear to seek referrals and it will make bitcoin getting down

I have no doubt that Facebook will accept bitcoins in future. The reason is very simple, Bitcoin is the future, so if not accept is the same as walking backwards.

If Facebook integrate bitcoin as one of their payment options, people would start having interest on it and may eventually adopt the system, seeing that it costs them a little less than using credit cards. They (Facebook) can do it, but apparently not at the current conditions of bitcoin. Wait another years (or even months, still depends though) and maybe they'll accept it.

Every large company will be forced to eventually.

Can be. They can really add another payment system if they want to. If bitcoin continue to get adoption, then maybe they will also integrate the system into their sites as a form of payment.

Not forced by laws or politicians, but forced to by market forces making them decide to accept bitcoin or go out of business in much the same way that all large businesses are forced to accept credit cards.

Force may not be the correct time, they will decide to do it to fullfill their client's needs and improve their services.

No doubt they will try their own custom solution. Reddit has suspended their plans but a similar solution may be adopted by Facebook.