Is following da’eef hadeeths regarded as shirk?

It cannot be said that acting upon weak (da’eef) hadeeths is regarded as shirk, because the hadeeths were narrated via chains of transmission (isnaads) and the scholars researched those chains. In many cases they found a man in the chain who was described as having a weak memory, but that does not necessarily mean that the hadeeth is fabricated, because we cannot accuse that scholar of telling lies and fabricating; but he may have made a mistake in transmitting the hadeeth or attributing it to someone, as happened in the case of many muhaddithoon who were described as being weak in hadeeth. It is permissible to act upon weak hadeeths subject to 3 conditions: 

1.That the weakness is not so severe as to make it likely that the hadeeth is fabricated.

2.That the hadeeth comes under a general principle, such as virtuous deeds.

3.That one does not believe when acting upon it that it is proven, so that he will not ascribe to the Messenger of Allah (blessings and peace of Allah be upon him) something that he did not do.

With regard to the differences between hadeeths, it may be due to a mistake on the part of one of the narrators; or it may be because some hadeeths abrogate others as is also the case with the Book of Allah. With regard to the idea that there are contradictions between the proven, saheeh hadeeths, this is not correct and cannot happen, because everything that has come from Allah, may He be exalted, or from His Messenger (blessings and peace of Allah be upon him) is free from contradiction, because Allah says (interpretation of the meaning): “Had it [the Qur’aan] been from other than Allaah, they would surely, have found therein many a contradiction” [al-Nisa’ 4:82]. So there are no contradicting verses in the Qur’aan and there are no contradictions in the proven hadeeths of the Prophet. 

But if the contradiction involves a da’eef or weak hadeeth, it does not matter, because we say that it is the mistake of one of the narrators. Some of the scholars regarded it as permissible to act upon da’eef hadeeth such as those that speak of virtuous deeds; i.e., these reports encourage us to do some acts of virtue. But if the weak hadeeth speaks of rulings, we cannot accept it. Hence Imam Ahmad (may Allah have mercy on him) said: When we narrate hadeeth concerning halaal and haraam, we are very strict; but when we narrate reports about acts of virtue, we are more lenient, because no rulings depend on reports about acts of virtue; rather they are like the virtue of fasting, jihad, prayer and so on. 

And Allah knows best.