In the specific case of Facebook, it is not about driving wages down. Facebook pays decent wages, even for Silicon Valley standards. It is about not increasing wages.
If it's not (in any way) about wages, then there would be no problem for Congress to repeal the 1965 Immigration Act in its entirety, cancel all the programs enabled by it, and (via the market) actively/aggressively solicit long-term unemployed US citizens in their place - as regular workers. There are more than enough of them to go around to be not only qualified, but very well qualified. Unfortunately, citizenship in the US makes people expensive, even for hard-working, by-the-book immigrants that want to come to the US.
Truth of the matter is, in the SF Bay Area, it is hard to be unemployed if you're a properly skilled tech worker, citizen, green-card holder or otherwise.
Truth of the matter is that "properly skilled" can be redefined to exclude otherwise-suitable US citizens too easily. In the eyes of an H1-b/L1/etc. supporter, "properly skilled" is equivalent to saying "has proper fear of an employer". If you were to go to the extreme end of business-friendliness (which spawned the H1-b preference), the ultimately qualified worker is a slave. They cost nothing and are the easiest to dispose.
That doesn't mean I condone the way that the H1-B program often is being abused today. I've seen abuse, and we'll always see that.
Then get rid of what enables the abuse - every single guest worker program. After that, strict enforcement of immigration laws already on the books - SB1070 and similar laws show that it works.
But this is only made possible due to the ridiculous limits on permanent resident visas vs the amount of H1-B visas, as I pointed out in this comment
The only proper limit for all guest worker programs is 0. If you want someone enough, they'll take up naturalization where they can't be corralled between sponsor employers. It might make them incur business-unfriendly "costs of freedom" (by being able to choose their employer), but the market also functions to raise prices.