Based on firewall logs I have seen, it probably happens all the time. Firewalls are constantly being probed daily, even in small companies. The reason you don't hear more about it is that big names, like Sony, are so recognizable, and get all the news. In addition, large companies tend to expose more attack surfaces to the Internet, since they want to provide more service. This makes them more vulnerable, and likely to be attacked, and to hit the news.
Small companies often do not have a dedicated IT staff, and have no proactive support, only reactive. In a case like this, I could see attacks going under-reported, and missed altogether. There is no one there to even realize they have been attacked...this assumes the hacker comes in and goes out quietly. Also, many small companies are not very good at patching or making sure their anti-virus is up-to-date and working well. Then internal computers get infected, and can give a hacker the keys to the door.
Although possible, I doubt that attacks, unless they are denial of service intended, are going to bring down servers. The hacker doesn't want that...most don't want you to know they have been there, and they very well may want to come back.