Fairfax County political analysts said yesterday that the relatively small margin of victory won by supervisor Nancy K. Falck in Tuesday's primary is a sign that no incumbent is safe from voters who are angry about the county's transportation and development problems.

"Nancy's was viewed as one of the safest seats on the {county} board, and {challenger Lloyd L.} Thoburn was the only vehicle for protest," said one Republican official. "People are upset {about development}, and Nancy was the only incumbent on the ballot, and they took it out on her."

"The message is that people who for the last few years never saw a building they didn't like are going to find themselves out of office after November," said Fairfax County Democratic Committee Chairman Harris N. Miller. "The theme of the Thoburn campaign was 'manage growth.' If I was a Republican, I'd be thinking: 'I'm in big trouble.' "

Falck, a Republican who has generally supported the county's development policies during her eight years as the Dranesville District supervisor, beat Thoburn, a religious conservative, 53 to 47 percent. In the Nov. 3 general election, she will face Lilla D. Richards, who won the Dranesville Democratic primary with 93 percent of the vote, and Robert L. Thoburn, Lloyd's father, who filed as an independent.

In contrast to Falck's margin of victory, lawyer and former White House associate counsel Bobbie Kilberg defeated conservative civic activist Joyce Sutcliffe with two-thirds of the vote in the GOP primary for the 32nd state Senate District from the same part of the county, emerging as a strong contender to Democratic incumbent Clive L. DuVal 2d.

Some have suggested that Kilberg, who spent almost $50,000 in the primary, may be positioning herself to capture DuVal's seat in 1991. DuVal, 75, who has served for 15 years, has said he will retire after one more term.

Kilberg "came from being a long-shot to being a contender," said J. Marshall Coleman, former Virginia attorney general and a prospective Republican candidate for governor in 1989. "There's something in the air. People are saying she's a real contender, that this is not just building for 1991."

"We need to be thinking longer-term about Bobbie Kilberg," said Don Beyer Jr., a leading Northern Virginia Democratic fund-raiser. "We have to keep her from being considered the heir apparent in 1991 . . . if she does well in 1987."

In the Dranesville Republican race, Falck had said before the primary that she hoped to receive "in the high 50s," while party officials said that less than 65 percent would be a sign that she is vulnerable in November.

Falck said yesterday that her margin of victory was comfortable and disagreed that voters supported Thoburn because they disapprove of her stand on the county's growth. She said her showing was not stronger because Thoburn outspent her almost 3 to 1 and sent out "trash" mailings that misled voters.

Falck said that Democrats also helped Thoburn by voting for him in the Republican primary because they thought he would be easier for Richards to beat in the general election, an explanation also offered by County GOP Chairman James Swinson. Miller denied that there was a large crossover vote, saying it would be unusual in a district where there was also a Democratic primary. Voters in Virginia do not register by party and can vote in either primary.

Political analysts give Thoburn's father, a former state delegate, little chance of winning in November and said his candidacy could have two effects: to draw votes from Falck, possibly giving Richards a victory, or to split any anti-Falck vote with Richards, giving Falck the victory.

Asked what effect Thoburn's independent candidacy might have in the general election, Falck said, "I think it will be a whole different scenario Nov. 3. Hopefully, people will realize that this is a personal vendetta on the part of the Thoburns because I wouldn't {vote to} rezone their property."

In the Springfield District, where former School Board member Toni M. Carney soundly defeated civic activist Beatriz Garcia in the Democratic primary for the district's board seat, local party officials are also concerned that the candidacy of Thomas E. Giska, who filed Tuesday to run as an independent, may split the party's vote and help incumbent Elaine McConnell, a Republican.

Daniel Edward Belsole filed to run as an independent in the Mason District against Supervisor Thomas M. Davis, a Republican who has no Democratic opponent.

James S. Morris and Robert T. Robarge filed to run as independents for the post of chairman of the Board of Supervisors, in which the fight between Republican John F. Herrity and Democrat Audrey Moore is expected to be the premier race of the season.

In three other Republican primaries Tuesday:Fairfax County schoolteacher Jane Woods defeated conservative lawyer Robert W. Beers in the 37th House of Delegates District with 62 percent of the vote. Lawyer and political newcomer A. Strode Brent narrowly defeated former state delegate Gwendalyn F. Cody in the 38th House District. Lawyer C. Ronald Smith defeated former School Board member Gerald Fill in the 36th state Senate District with 54 percent of the vote.