Both companies are more interested in the talent than the tech
Google's not short of cash so the fact that it leads the pack in acquisitions is no surprise. But Google is not short of people wanting to work for it either, yet most of these acquisitions were about the talent and not the company or technology being acquired.The same goes for Facebook, which bought 12 companies in 2012, exactly as many as Google. Five of Facebook's acquisitions were also about the talent.
The only surprising thing is that not all of them were. Facebook has very rarely bought a company and kept it running. The big exception, of course, is Instagram for which Facebook paid a pretty penny.
Note that Google has slowed down considerably in 2012 as it acquired several times more companies in the year before.
Facebook is not short of cash either and if it can't use case, it can use its stock, especially now that it's starting to gain some value again.