My CS department greatly lacks invited talks hosted by faculty members and/or seminars organised within/by the department. Except for the seminars done by graduate students as a requirement for their degree, during the last year there was no seminar organised by the department/faculty member. If curious, the department size is relatively small (around 20 faculty members) and some faculty members are well-known figures in their areas. As a PhD student I like seminars and wondered how to address this to the department.
It's hard to change the culture of a department as a student. You should definitely tell your advisor, and the department chair, that you wish there were more lectures. You might also talk to other students and encourage them to express their feelings if they feel the same.
Then you should take action. Is there a student chapter of the ACM at your school? If not, organize one. Then organize monthly seminars by inviting faculty and students in your department and from nearby schools to present their work. If you can get a little funding, invite one of the world leaders in your area to give a seminar at your school. You might be able to get funding through the department or through the ACM (I know SIAM gives funding for such things).
It requires work, but it sounds like it will be worthwhile to you. It is also a great excuse to meet important people in your field.
We have somehow the same problem (not that we don't have seminars - but we don't have enough in my opinion). The graduate student association in our department (CS as well) tried once to organize such seminars. There were many interested professors and students, especially when it comes to topic that are not very technical [a professor told me that such lectures are more useful for the students].
However, to make these seminars frequent, it was found that the best thing is to do is to make the seminars specific to one topic. In order to do this, there is a need for a group of researchers led by a professor (or more) - all working on the same area. Some professors tried to do something like that, however, only the big labs were successful in this mission (i.e. labs with many professors and students). Issues are:
1) finding large number of speakers,
2) intellectual property, some researchers simply do not want to share their ideas with others. [in fact, the association tried to do a conference each year, but the fear of IP theft was the biggest obstacle and the lack of motivation for researchers to publish in an non-indexed conference].
Conclusion, you need a lot of support in order to organize frequent seminars in your department, but it is not impossible. Talk to young professors, they are the most motivated.
Your faculty isn't huge but, at the same time, it's bigger than many research groups that have successful seminar series.
The first thing you need to do is to establish why there aren't already seminars. If the reason is just that nobody has bothered to organize any, you can deal with this; if the reason is that the faculty members aren't interested in attending seminars, you might be wasting your time.
One important piece of information that's lacking is your location. If your university is in, say, the middle of North Dakota, it's going to be difficult and very expensive to organize a seminar series with external speakers. Every speaker will need flights and a hotel and, even when you pay expenses, speakers are unlikely to take a 2-3 day trip just to give a seminar to 20-30 people. On the other hand, if you're in, say, England, and there are several other universities within two or three hours' travel from you, things are much more practical.
A good way to get the ball rolling might be to organize informal lunch-time seminars. Start with the grad students and try to get a few faculty on-board. The idea would be to have a 30-45-minute whiteboard talk while people eat lunch. No slides, so people don't have to spend hours preparing; lunch-time so it's not taking time out of people's days; typical topic would be "What I've been working on recently", so things stay relevant. Once you have a seminar culture started, you can think about doing something bigger.