TweetDeck, a popular app used to view and post Twitter messages, was restored to service Friday night, hours after a TweetDeck user said he was able to improperly access hundreds of Twitter accounts.

Twitter, which owns TweetDeck, pulled down the service on Friday after the TweetDeck user, Geoff Evason, posted a message on Twitter showing a digital image of what appeared to be access points to dozens of other Twitter users and some Facebook accounts.

"Hey, @TweetDeck: A bug in your software has given me access to hundreds of accounts," Mr. Evason posted. He later wrote online that he logged into TweetDeck and suddenly saw he was able to post to Twitter from many accounts that didn't belong to him.

In an email message Friday night after TweetDeck was restored, Twitter explained that it discovered a "bug" that Twitter said caused a "very small number" of TweetDeck users to be able to access other TweetDeck user accounts. Twitter said account passwords weren't compromised, and the company said it didn't know of situations where account access was used "maliciously."

UPDATED 3/31/12: On Saturday, Facebook said the TweetDeck error affected fewer than 250 of its users, and the company said there was "no abuse of these accounts or our systems." (People can link their Facebook accounts to TweetDeck, and use the software to post both to Twitter and Facebook from the TweetDeck dashboard.) Facebook said it was working with Twitter to "understand the full scope of this issue" with TweetDeck.