Authenticity of Hadith about Lying
Question

"When a believer utters a lie without a valid excuse, he is cursed by seventy thousand angels. Such a stench emanates from his heart that it reaches the sky and because of this single lie Allah writes for him a sin equivalent to that of committing seventy fornications, the least of which is fornication with one’s mother."
I want to know; is this hadith about frequent liars or liars as a whole? If this is for liars as a whole, then it means lying is greater sin than fornication? Isn’t it?
Every day we come across people who tell lies. So we can say that they have committed 70 fornications. From this point of view, I don't think so that there is any single Muslim who does't lie. Does that mean each and every single Muslim who lie without valid excuse has indeed committed 70 fornications?
Lying is something which Muslim can avoid as possible as he can, but he can't completely abstain himself from lying.
Praise be to Allah.Lying in Islam
Lying in Islam is one of the characteristics of the hypocrites , and it is a blameworthy trait which encourages one to commit evil and prevents one from doing good. Al-Bukhari (6094) and Muslim (2607) narrated that Ibn Mas‘ud (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “I enjoin you to be truthful, for truthfulness leads to righteousness and righteousness leads to Paradise. A man may continue to tell the truth and endeavour to be truthful until he is recorded with Allah as a speaker of truth. And beware of lying, for lying leads to wickedness and wickedness leads to Hell. A man may continue to tell lies and endeavour to tell lies, until he is recorded with Allah as a liar.”
Worst type of lying
The worst kind of lying is telling lies about Allah and His Messenger (blessings and peace of Allah be upon him), then spreading lies among people in order to cause trouble between them. Habitually telling lies when speaking is one of the characteristics of the hypocrites, as the Prophet (blessings and peace of Allah be upon him) said: “There are four characteristics, whoever has them all is a pure hypocrite, and whoever has one of them has one of the characteristics of hypocrisy, until he gives it up: when he is entrusted with something, he betrays that trust, when he speaks he lies, when he makes a covenant he betrays it, and when he disputes he resorts to obscene speech.” (Narrated by al-Bukhari (34) and Muslim (58)
How to repent for lying in Islam
What the one who has lied must do – even if he told a single lie – is repent to Allah, may He be exalted. At-Tirmidhi (1973) narrated that ‘Aishah said: No characteristic was more hateful to the Messenger of Allah (blessings and peace of Allah be upon him) than lying. If a man told a lie in the presence of the Prophet (blessings and peace of Allah be upon him), he would continue to feel some unrest in his heart until he knew that the man had repented. (Classed as sahih by al-Albani in as-Sahihah (2052)
Authenticity of hadith about lying
We could not find any source for the hadith mentioned in the question, after researching the matter. What appears to be the case is that it is falsely attributed to the Messenger of Allah (blessings and peace of Allah be upon him). The scholars stated that one of the signs of a fabricated hadith is that it mentions an exaggerated reward or punishment for a particular deed, and this hadith is like that.
Is lying worse than zina?
The basic principle is that zina is worse than telling lies , hence sins are mentioned in decreasing order of gravity, after the sins of ascribing partners to Allah (shirk) and murder, in the verse in which Allah, may He be exalted, says (interpretation of the meaning):
“And those who invoke not any other ilah (god) along with Allah, nor kill such life as Allah has forbidden, except for just cause, nor commit illegal sexual intercourse and whoever does this shall receive the punishment” [al-Furqan 25:68].
With regard to zina , there is the prescription of punishment in this world (the hadd punishment), and the warning of punishment in the Hereafter, such as is not mentioned in the case of lying. 
When can lying be worse than zina?
However, some kinds of lying are worse than zina and other major sins, such as telling lies against Allah and His Messenger (blessings and peace of Allah be upon him). Allah, may He be exalted, says (interpretation of the meaning):
“Say: ‘Verily, those who invent lies against Allah will never be successful’” [Yunus 10:69]
The Prophet (blessings and peace of Allah be upon him) said, in the mutawatir hadith: “Whoever tells a lie against me deliberately, let him take his place in Hell.”
With regard to saying that lying as a whole is worse than zina, this is incorrect, let alone saying that it is worse than committing zina seventy times (“seventy fornications”), the least of which is fornication with one’s mother. This is among the silliest of notions and most obvious of lies, and is impossible!
Authenticity of the hadith ‘Can a believer tell lies?’
With regard to the report narrated by Ibn ‘Asakir in his Tarikh (27/241) from ‘Abdullah ibn Jarad – according to which he asked the Prophet (blessings and peace of Allah be upon him: O Prophet of Allah, can a believer commit zina? And he said: “That may happen.”
He said: Can a believer steal? He said: “That may happen.”
He said: Can a believer tell lies? He said” No; rather lies are fabricated by those who do not believe”–
This is a false hadith. Its isnad includes Ya‘la ibn al-Ashdaq. Ibn Hibban said: They fabricated hadiths for him and he narrated them without realizing. Abu Zar‘ah said: He is nothing; he does not tell the truth. (Mizan al-I‘tidal, 4/475)
It was mentioned by al-Albani in ad-Da‘ifah (5521), where he said: It is da‘if (weak).
Something similar may be said about the hadith of Safwan ibn Sulaym: It was said to the Messenger of Allah (blessings and peace of Allah be upon him): Can a believer be a coward?
He said: “Yes.”
It was said to him: Can a believer be a miser?
He said: “Yes.”
It was said to him: Can a believer be a liar?
He said: “No.” Narrated by Malik (3630). It is mursal, and was classed as da‘if by al-Albani in Da‘if at-Targhib wa’t-Tarhib (1752).
For more, please see these categories: 
Bad Behavior
Sins of the tongue 
And Allah knows best.