One thing that always chafes my hide is when I get an invitation to something fun-sounding, then open it only to find that I have to pay to attend the fun-sounding thing.
The use of web-based invitations to invite people to pay-for-it events seems hardly offensive -- I mean, after all, we get event invites (via email or snail mail) that require us to pay, right? The Benefit Viewing Party of "Borat," or the Benefit Concert for Epileptic Dogs.
I think it's the way these events announce themselves that make the difference. When I see an email with the subject line "You're invited!" I automatically think "party!" not, "something I have to pay for!"
And, web-based invite programs do offer a "pay with paypal" option. These free programs have to monetize somehow, and I get that. So does that kill off the impolite factor? I don't think so.
I'm sort of on the fence. I looked back to pay-event invitations that didn't offend me, and they were worded better (e.g. - "Join us for a lecture and lunch") -- and I think that's what I'm looking for - more "truth in advertising." "Join us" doesn't imply that you're being invited gratis; you read on and have the option to pay and go, or not go.
The way I was raised, when you invited someone to something, that meant s/he was to be your guest.
What say ye, my amazing readers?