In the context of OnlineCommunities, TombStoning is the usurption by another individual of a no longer participating author's identity. As far as the rest of the community knows, the two people are the same as long as the identities are difficult to confirm.
For instance, if someone took a very old namepage from WikiWiki whose author had written very little, it would not be difficult to take on that identity falsely without anyone noticing.
UseRealNames has an advantage and a disadvantage. On one hand, it's harder to usurp someone's real life identity than a pure pseudonym as real names have multiple means of verification in the RealWorld, whereas PenNames are pure fiction and thus have very few means of verification (cf WalledIdentity) . On the other, if someone does usurp your identity and then misuses it, it may be difficult to demonstrate that you weren't responsible for the actions attributed to your signature.
Of course, this is nothing new on the Internet. Who hasn't seen a FakeMail attack on a mailing list?
Passworded logins are often used to try to defend against this, as in theory only the original user will know the password. However, this may fail AvoidIllusion: if the password is guessable, then the tombstoner can abuse people's misplaced faith in technology. An alternative attack is to choose a new login that is similar to the old one and claim to have forgotten the password to the old attack. Compare LoginsAreGood?, LoginsAreEvil.