Only introductions made through friends help more people in the US to find love.
But dating sites' claims that they employ "science-based" methods of bringing lonely hearts together cannot be trusted, say scientists.
A review of more than 400 psychology studies and surveys found there was no longer any stigma attached to internet dating.
According to one industry estimate, dating sites attracted 25 million users worldwide in April last year alone.
The researchers said internet dating was a great help to single people in Western societies, for whom matchmaking was "grossly inefficient" after leaving school or university.
However, they warned that online love had serious pitfalls for the unwary.
Professor Harry Reis, from the University of Rochester, US, one of the study's five co-authors, said: "Online dating is definitely a new and much needed twist on relationships.
"The internet holds great promise for helping adults form healthy and supportive romantic partnerships, and those relationships are one of the best predictors of emotional and physical health."
But he cautioned that comparing large numbers of potential dates may encourage a "shopping mentality", leading people to become over-picky and focused on narrow criteria such as attractiveness or interests.
Corresponding by computer for weeks or even months before meeting face-to-face could also create unrealistic expectations.
Online sites may also encourage fruitless and ultimately destructive quests for "perfect match" soul mates.
In the early 1990s, fewer than one per cent of the US population met partners through printed personal ads or dating agencies, Prof Reis and his colleagues say.
By 2005, 37 per cent of single American adults who were internet users and seeking a partner had dated online.
The latest research, by Professor Michael Rosenfield, from Stanford University, California, showed that between 2007 and 2009, 22 per cent of heterosexual couples and 61 per cent of same-sex couples had met on the internet.
Those figures were likely to be greater today, said the review authors. Online dating had become the second most common way for couples in the US to meet, after meeting through friends.
The authors wrote: "Online dating has entered the mainstream, and it is fast shedding any lingering social stigma."
No independent published studies existed that could explain the "science-based" approach offered by dating sites, they added.
Sites kept their "in-house" research methods and data collection secret, and it was therefore "not verifiable" by outside parties.