There's pressure for things to turn romantic quickly.
It is well documented that physical attractiveness is a major factor in romantic attraction, especially initial attraction (Sprecher, 1989).
Not surprisingly, physically attractive people are more successful at online dating (Hitsch et al., 2005).
I found that those who met their partners via online dating sites became romantically involved significantly sooner (an average of two-and-a-half months) than those who met in other ways (an average of one-and-a-half years). Typically, they post old photos from when they were 100 pounds lighter and 10 years younger, or they post photos that hide their body shape, which is not just a physical attribute, but a commentary on their lifestyle.
This suggests that online dating sites don’t facilitate slowly finding love the way that we often do offline. As mentioned earlier, those who are introverted or shy may find online dating more palatable than other ways of looking for love. I've had more than a few claim to love physical fitness and healthy eating, only to confess upon meeting, at which point it becomes obvious, that they actually do neither.
But in real life, after we get to know someone and like their personality, we begin to find them more physically appealing as well (Kniffin & Wilson, 2004).
Making a quick decision based on an online photo doesn’t allow for this slower development of physical attraction and may cause us to dismiss potential mates to whom we could become attracted.
There are dating sites devoted to particular religious groups, like Christian Mingle or JDate, for example, as well as sites that cater to gay and lesbian daters. This removes a lot of the ambiguity that you face when you meet an interesting person at a work event or a party. Because of the ease and relative anonymity of online dating sites, we may take more risk reaching out to people we would not approach in person.
And even though men generally contact women more than vice versa on these sites, research has shown that a sizable minority of women do reach out to men they find desirable online, suggesting that these sites allow some women to overcome traditional gender norms that cast them in a passive role of waiting to be approached (Scharlott & Christ, 1995). Shy or socially anxious individuals often have difficulty forming and maintaining close relationships (Alden & Taylor, 2004; Davila & Beck, 2002).
In addition, when we read vague information about someone, we mentally fill in the blanks with specific details that may be incorrect (Norton & Frost, 2007).