Depends on the expected magnitude of the effect. If small enough, you're still good. There are also ways to mitigate those effects, such as lurking only, never tag anything, use the "like" wisely…
It's not just about shadow accounts, by the way. There are more direct effects. For instance I was once forced to set up an account for logistic reasons (they used Facebook extensively, if not exclusively). Worse, when I tell them I didn't got some news (because I didn't check that account very often, and I turned off the very annoying notifications), they say "but I sent the mail" (they only used Facebook). I have since "deleted" my account, good riddance.
So, the effect on me was direct and significant. Depending on how you use Facebook, you can have a similar effect. The worst you can do is set up events and invitations on Facebook only, forcing your friends to either use Facebook, or drift apart.
It's not just about shadow accounts, by the way. There are more direct effects. For instance I was once forced to set up an account for logistic reasons (they used Facebook extensively, if not exclusively). Worse, when I tell them I didn't got some news (because I didn't check that account very often, and I turned off the very annoying notifications), they say "but I sent the mail" (they only used Facebook). I have since "deleted" my account, good riddance.
So, the effect on me was direct and significant. Depending on how you use Facebook, you can have a similar effect. The worst you can do is set up events and invitations on Facebook only, forcing your friends to either use Facebook, or drift apart.