I wouldn't call SETI an experiment. Its a "look and see." I'm not criticising it, mind you. A lot of good can come from just looking around. Really, you can't experiment until you've looked around enough to get a general idea of the situation. Seems like a lot of branches of science have this sort of vaguely directed investigation in their past.
is SETI a scientific experiment? is it falsifiable? is the falsification simply a lack of alien messages? is it repeatable?
In the context of what it's trying to test, yes. SETI isn't trying to answer the question "is there intelligent life beyond your Earth?" It's trying to answer the question "can we detect intelligent signals from beyond the Earth?"
If it turns out we can't detect them, then we can't. That could either be because we're too stupid to do so, or because they aren't there to detect. Either answer need have nothing to do with the actual existence of extrasolar intelligence.
Well, I would say there's a third possibility: The question is too difficult to be answered.
The SETI people acknowledge this problem: They are using a single radio telescope, looking over a very small part of the radio spectrum, and only for a certain type of signal.
Think of it this way: We've hidden a secret message in plain sight somewhere in the United States. The only tool you have to find it is a metal detector. That's great if the message happens to be written on something metal, but if it's written using logs on the beach, you'll miss it...assuming you even get to that part of the country in any reasonable amount of time. It isn't that you're stupid...it's that there are so many ways to have the message out there that unless you look in the right way in the right place, it'll pass you by.
The SETI project is a fishing net with a really big mesh. We'll be lucky if anything happens to get caught. If we don't get anything, as crash said, it says nothing about whether or not there was anything to catch.