By the end of 2010, the pro-business party had 68,541 members – roughly 4,000 fewer it had in 2009, according to a report in the Frankfurter Rundschau newspaper on Thursday.
Although official figures for 2011 are not yet available, even more members have left the FDP this year, party spokesman Wulf Oehme told the newspaper.
“We can't see a dramatic improvement or brightening trend,” Oehme said.
Only the party's chapter in Saarland saw a membership increase last year from 1,826 to 1,830.
The paper reported that even large businesses normally inclined to donate to the FDP were now shunning the party. Oehme acknowledged that part of the problem could be poor showings in recent state elections.
"Naturally bad election results, fewer members and, fewer donations hurt our finances," he said, but denied the party was "listing dramatically."
The FDP misfortunes have been closely tied to those of its outgoing party leader, Foreign Minister Guido Westerwelle, who has said he will give up post as chairman later this month.