Facebook and Instagram are back online as of early Thursday morning, but the reasons for their longest outages ever on Wednesday remain unexplained.

Both services were unavailable for much of the day. Facebook, on its status page, said it was experiencing "increased error rates" after 11 a.m. Co-owned service Instagram acknowledged its users were "frustrated" at the outage and said its team was working to resolve the issue.

Facebook's WhatsApp application was also affected by the outage.

In a tweet later Wednesday afternoon, Facebook said its issues were not related to a denial of service attack.

But 13 hours later, all was back to normal.

Instagram tweeted a meme of Oprah Winfrey captioned "anddd...we're back" at 12:41 a.m. to announce its return.  Facebook, on its own page, posted "it's good to be back" at 1:11 a.m. with a smiley face emoticon. The problem was not closed out on the status page as of 5:45 a.m. on Thursday.

Users of both services turned to Twitter for confirmation of a problem, and #facebookdown and #instagram were top trending hashtags all day long.

It has been a rough week for several digital platforms. Verizon had issues with texting on Tuesday morning due to a "wireless network issue." Google users had problems with Gmail and Google Drive late Tuesday night, which were resolved early Wednesday morning.