First minister and DUP leader Peter Robinson's future in politics is under threat after he lost the Westminster seat he held for 31 years.
Alliance deputy leader Naomi Long is the new MP for East Belfast.
Both main unionist leaders have suffered defeat in the election as Sir Reg Empey lost in South Antrim.
Sinn Fein's Michelle Gildernew was returned in Fermanagh/South Tyrone after beating Rodney Connor by four votes in a third recount.
After the second recount, Mrs Gildernew, Sinn Fein, was just two votes ahead of independent unionist unity candidate Rodney Connor. She had 21,300 votes to Mr Connor's 21,298.
With all seats declared in Northern Ireland, the DUP have eight, Sinn Fein have five, the SDLP have three, the Alliance Party has one seat and one seat has been won by the independent, Lady Hermon.
Meanwhile, despite his party's pact with the Conservatives, Ulster Unionist leader Sir Reg Empey has lost to William McCrea, DUP, in South Antrim.
Senior Ulster Unionist David McNarry said Sir Reg was "finished" as party leader. But Mr McNarry said he was "not in the stakes for leadership", turning the focus, instead, on unionist unity.
BBC NI political correspondent Martina Purdy revealed that later on Friday the UUP party officers are due to meet for a post-mortem on the election.
There is speculation about whether Sir Reg Empey will continue as leader but so far there has been no comment from him.
In a shock turnaround, DUP leader Mr Robinson's vote fell from a 6,000 majority in 2005. This time, he took 11,306 votes and Ms Long won with 12,839 votes.
His wife, Iris Robinson's resignation and questions over expenses and land deals may have played a role in his defeat.
Speaking after the result was announced, Mr Robinson said he would have "preferred not to stand" in East Belfast.
"I have a job to complete with my mandate at the Assembly and I will continue to carry out that important work," he said moments after the shock result was announced.
Ms Long said her election was a positive message from the people of East Belfast
"They want stability and they want peace and I think that those are positive messages coming not only from people who voted for me but also those who voted for the former incumbent in that seat they want to move forward," she said.
"I think we need to take that very seriously and make sure that's exactly what we deliver."
BBC NI political editor Mark Devenport said the shock defeat "damages Mr Robinson's stock".
"None of the DUP MPs who we have been speaking to on our election special wishes to be Brutus wielding his dagger, but the results increase the standing of the Paisley and potentially the Dodds dynasties in the party.
"So who will be in charge of any negotiations with the future would-be Prime Ministers?"
In better news for the DUP, in Antrim North, Ian Paisley Jnr polled 19,672 votes, winning the seat comfortably despite the contest from TUV leader Jim Allister.
Overall, the TUV polled badly and did not cause significant damage to the DUP vote.
And the Ulster Unionists' pact with the Conservatives has not produced the fightback they had hoped for.
Sinn Fein leader Gerry Adams has retained his Belfast West seat polling 22,840 votes
Bairbre de Brun, SF, said the recent bad publicity surrounding Gerry Adams has not had any effect on his vote. The turnout in his constituency is down by almost 14% but Mr Adams' share of the vote is slightly up.
The SDLP saw off the Sinn Fein challenge from Stormont Education Minister Caitriona Ruane in South Down. SDLP leader Margaret Ritchie polled 20,648 votes, taking over the mantle from party veteran Eddie McGrady.
In Londonderry, the count was suspended after a pipe bomb was left at the Templemore sports complex. But it later resumed and Mark Durkan was returned as Foyle MP and Gregory Campbell returned as East Londonderry MP.