There is a problem with attempts at making online dating work like
"real world" dating. I'll state my reasons for this belief up front:
I'm very happily in an almost two year relationship with somebody I've
met online (OKCupid) whom I could have never met in person. Before,
I've had a happy (even if not lasting) and amicably ending
relationships with other women whom I could have never met in person.

I'll use my anecdote as one archetype ("geek of one type, seeking a
geek of another") for whom online dating works very well. When I started
dating, I was 23 and working full time while atteding graduate school,
living in San Jose. Most of my fellow graduate students were already
in relationships. The idea of work place romance seemed scary to a new
comer both to dating and the work force. I had a very small and
limited friends circle.

My career, educational and financial situation meant that even if I
had time for social activities (I didn't), I'd have no ability to
relate to a (statistically) random person my age, irrespective of
their gender. They didn't have a stable job or attend graduate school,
they didn't live on their own.

The latter paragraph makes me sounds extremely shallow, but consider
this: it's not a good idea to have a [1] long term relationship to
somebody earning significantly less or significantly more than
you. For example, simple things that involve a money/time trade-off can
turn into cultural conflicts: for example, hunting 30 minutes for
street parking when you could pay $10 to park in a public parking
garage (or in a city, taking the bus instead of a cab). Dating somebody living with parents is difficult if you live alone; dating with somebody who expects you to go to happy hour when you're in class or working is also difficult.

([1] Another factor: I wanted a long term relationship at 23. Most men
my age didn't, leading women who were to almost automatically infer
that I "just wanted to get laid").

Finally, I also wanted somebody who was intellectually inclined. While
(for reasons I can't comprehend), most men are intimidated by
intelligent women, I wanted somebody who could be a great conversation
partner; who could discuss (non-computer) science, philosophy, music
and literature. I just didn't want to hide my geekyness from my
partner.

Online dating worked well in this way: I could meet women living in
San Francisco and Berkeley (which have a higher concentration of
single geek-o-phile women than San Jose), places where I wouldn't
"just spontenously" make a trip (given the distance). I only had to
send a few messages, as I could perform intelligent searches based on
match percentage and then further narrow down the results by actually
reading the profiles (something most men on dating sites don't quite do).

In some ways, you could argue I was engaging in what Adam Smith called
"hypergammy". In many cases, I felt the women I met were "out of my
league". However, that's completely silly: how can you know if you're
out of somebody's league, if you don't know what sport they're
playing? In fact, one woman whom I considered "out of my league" also
told me she considered me "out of her league".

Are there problems with online dating? Yes, of course. If you're curious to what that problem is, ask a woman who actually used an online dating site about what is in her mailbox. Fortunately, even a simplistic ranking algorithm (TF/IDF on the message based with the keywords of the recipient's profile to burry the generic "YO GURL UR SO HOT" messages -- from men who never bothered to read the profile of women they messaged) can help greatly reduce this. Helping people discover relevant content is a problem of algorithms (ranking content and finding meaningful patterns in it), systems programming (building the systems that could store, modify, retrieve and process the content in real time as opposed to in batch) and user interface (there's a whole discipline around this). OKCupid and EHarmony (I've never used it, but they happen to use an open source project I contribute to) are examples of at least getting some parts right. Problem is that they couldn't go outside of their niche (geeks in the case of OKCupid and -- in my perception, reality may be different - traditional, usually conservative/religious and heterosexual[2] people in the case of eHarmony).

([2]: I realize that this post is very"heteronormative", my apologies for this)

In short, online dating should really be called "online discovery of
people you can date". Real dating happens in person and the same rules
(e.g., "if you're a guy be nice, pay and open the door") apply. It's
just that you're exposed to people you might not be exposed to
otherwise (and you feel more confident when on a date with them: you know they're actually interested in you). Each side discovers something: I discovered that being a
programmer (doing something I have a deep passion for, doing creative
work) wasn't something I needed to hide and play down, while my dates
discovered that programmers can also have an interest in music,
neuroscience, philosophy and literature.

In short, making online dating based on offline social networks makes it nearly useless. Given the experience of Myspace and Friendster (as opposed non-dating based social networks like Facebook), it's a non starter from the get-go: male friends spamming your female friends ruin the "social network" part, the social network parts ruins the chance of any dating that wouldn't happen in real life happening due to the use of the site (why, then, use the site in the first place?)