I'm with W130SN, if it's public google can get access to it. To me a private network is that, private, no one else has access. That is almost impossible to pick up, a public network service is different, google can submit 100's of sites for links, think about it, it cost them pennies. The volume of sites they can submit means they can catch a fair size chunk of the network, then they can back trace a few things and do real damage. I'm not saying don't use these services, most work great, be aware of what is going on. If you have one site, you need to spend money building your own, if you don't looking in the mirror shows you who is to blame if the public network gets hit. Public networks are ideal for churn and burn type sites, they will bump you up and away you go. If were doing serious stuff for a client who has one site, they'd be building their own network.
Regarding the red flag, it probably means any service advertising they have no footprints will be a high target to prove them wrong, not hard G signs up, adds plenty sites, does a bit of work, bang, another network out.
