I recently started online dating, signed up to POF and so far chatted to a few but got nothing, a month ago I signed up to Match and have had three meetings so far.
I treat the whole thing as a a means to meet new people as possible friends and find out about them, nothing more, as for the whole "spark" thing, for me it means that you find something about the person that peaks your interests enough that the time flies by, the conversation flows easily and you feel at ease/comfortable with the person, that being said, my third meeting was only yesterday, lasted 5 hours and was great, and think there will be a second date coming up with her.
If you don't go into it expecting to meet Mrs Right, keep an open mind, be positive and honest, and above all try to enjoy yourself, then at the very least, you will have met a new individual, learnt interesting things from them and had a good time out and if your lucky, you may eventually meet a person that you both want to meet again and with any luck start a good relationship with.
Note: all the above is my own opinion and I am in no way a player or such, this is just my way of seeing it, having fun and see were it leads!