Facebook's location in Menlo Park is at risk for being targeted because most offices are at risk. Bombs threats happen, but that being said, the Menlo Park location is no more at risk of being targeted than any other set of building. Those who think bombing people is okay will choose any place with a large number of people inside.
Menlo Park holds a lot of prominent businesses, not just Facebook. I suppose this area could be targeted for many things. Everyone knows there is a lot of money within that city. I suppose it would depend on who is doing the targeting, which the question asker fails to identify.
Any high profile corporate headquarters is a potential target for terrorism. Striking at Facebook would be worldwide news and have worldwide repercussions. If they could disrupt the site somehow, that could cause global chaos and fear. Since most terrorists seek attention, striking at the Facebook headquarters is one potential way to gain it.
No, Facebook's location in Menlo Park is not at risk for being targeted, any more so than any other location where Facebook could be located. They will always have to be vigilant, because they have so many people who depend on them. But they have security, and it is a relatively safe area.