It depends, really. If a casual conversation sparks up between an old friend via Facebook or some shyt, I can understand how the potential of accidental flirting might take place. What really matters is if your partner can identify it and stop it in it's tracks. If your partner is actively contacting that individual and initiating those conversations however, it's a line crosser imo.
It's a slippery slope to navigate. I can imagine if one was militant about all online flirting as taboo to be a major problem. That's that type of shyt that leads to "You clicked like on her Instagram photo, you ain't faithful," which is