Oh, I know.
You can see it when they say "yeah man, I'll be there" that they aren't going to show. It's easy to tell what people are going to do by the way they respond. The non-commital people are thinking "oh god, I gotta be to work/school/whatever the next day..." or whatever would interfere with going out that paticular night. They probably won't be there, just because they won't be able to justify going out and taking a chance on a band they've only heard of (if at all).

Now, if you can go out and proove you're a fun person, then maybe a few will show. If you hold up and they have a good time then they hopefully sign the mailing list, so atleast you have people who want to know where you're playing. If you're really lucky you can get them to drag their friends along. It is a building process, but it's kinda satisfying... "hey, 15 people showed up, but 9 signed the mailing list... atleast 9 of them want to know the next time we're playing."

I have one person on our mailing list that I hear back from every time "Have you tried sending your demo to __________" That's some cool shyte if you ask me.