"We look at all leads, including Facebook," Swedish Security Service (Säpo) spokeswoman Maria Svensson told AFP on Friday.
Investigators have said they are "98 percent certain" the bomber is Taimour Abdulwahab, who grew up in the Middle East and became a Swedish citizen in
Säpo security chief Anders Thornberg could not say if Facebook had enabled the agency to determine if anyone had helped the bomber who blew himself up in central Stockholm on Saturday.
"It's too early to say and even if I knew, I could not answer because of technical reasons related to the investigation," Thornberg told daily Dagens Nyheter on Friday.
According to the daily, which quoted specialised Israeli website Haganah, Abdulwahab had one British-based Facebook "friend" who was friends with six of Samir Khan's Facebook friends.
Samir Khan, a US citizen, is believed to be behind an English-language al-Qaeda magazine and a pro-jihad website which has called for the head of Lars Vilks, the Swedish artist who drew the prophet Muhammad with the body of a dog in 2007.
Abdulwahab -- who according to media reports came to Sweden from Iraq as a child and blew himself up the day before his 29th birthday -- had been living for the past few years in Luton, northwest of London, with his wife and three children.
He blew himself up near a busy pedestrian street early Saturday evening, killing only himself, but two people were slightly injured when his car exploded a few minutes earlier about 300 metres away.
According to a prosecutor on the case, he intended to kill as many Christmas shoppers as possible, but may have had a faulty bomb.
Police are seeking to determine what exactly happened on Saturday, how he became radicalised and whether he had any accomplices.