SCO made the threat because companies were using Linux code under what they claimed was an invalid license. It's very unlikely there would be any grounds for suing someone who uses a website.

Not only that, you'd have to find their customers. I'm going to guess that just asking them "Hey, who are your customers?" won't be very productive (and, if they do answer because they want to brag, since they're plain ol' liars, they'd probably make up some really impressive sounding customers; big names who've actually never even heard of the company).

Of course, contacting their customers (if you can identify them), to let them know that the service is a crappy imitation of the real deal might be a useful pursuit. Maybe. One would have to be very careful about the tone of the letter.