You could do the "not yet open for business". That's a little rude.
The polite thing to do is say, "Great! That product isn't ready yet. We'll contact you when it is. What's your email?"
Or you could say, "Hey, we're still working on this. Could we contact you to find out what's most important to you about this product?"
Another option is just to flash an error message and redirect them back to Facebook or Google or wherever they came from. They will pretty much instantly forget about you: things are broken all the time on the web. That's kinda lame, though, so I'd generally lean for something that invites further engagement.
The polite thing to do is say, "Great! That product isn't ready yet. We'll contact you when it is. What's your email?"
Or you could say, "Hey, we're still working on this. Could we contact you to find out what's most important to you about this product?"
Another option is just to flash an error message and redirect them back to Facebook or Google or wherever they came from. They will pretty much instantly forget about you: things are broken all the time on the web. That's kinda lame, though, so I'd generally lean for something that invites further engagement.