All good questions! I've done online dating many times in the last 5 years or so and each time yielded different results. To answer your questions:
-Did you like online dating? Did you meet your significant other that way?
I think there are pros and cons to online dating. I found it was a great way to get introduced to a lot of people that you wouldn't have normally met. It's nice to have a lot of different guys that you can kind of pick and choose the ones that you think will match your values and interests. On the other hand, there's always the risk that they aren't being upfront and honest about themselves...but then it's always a possibility to come across that in "everyday life" so to speak. As for the second question, I met my current bf of 5 months on plentyoffish. I met the last guy I dated on the same site. You're going to find awesome guys and some crazy ones as well...but that's going to be the situation no matter where you meet someone.
-Is it okay to see multiple people at once?
I definitely think so, to a point obviously. Different people will feel comfortable with different things, but for me, I kept my options open until I was sure I wanted to pursue one particular guy. I think, if you're just casually dating, there's no reason not to hang out with other guys. No need to jump into anything. Just make sure you're honest about what's going on and that you and each guy you date are on the same page about the seriousness of things.
-How long did it take you to realize that you found someone you wanted to commit to?
It took me a couple of weeks to realize that my bf was the guy I wanted to be with. We took things slow, dated other people, and were really casual the first few weeks of dating. You will know if any one particular person is the right one to pursue, but I would suggest taking things as slow as possible. Especially having just left a long-term relationship. It was about a month into dating that my bf and I became "official".
-How did you dress for second/third/fourth dates? First date I did kind of casual, jeans and a nice top. Should I go fancier for the next date if we're going out to dinner, or should I do about the same?
Personally, I would just dress for the occasion. I don't think there's an increasing/decreasing level of dressiness or cuteness that goes with each date. If you're going to a fancier restaurant, maybe dress up a bit more. Our first date was at Starbucks, so obviously I went pretty casual. The next date was at a nicer restaurant, but I still just wore jeans and a cute top. Wear whatever makes you feel confident and is appropriate for the date.
Good luck with everything!

. I think it really depends on what you're doing. Too casual would definitely be sweats etc. Too dressed up would be something you would wear to a wedding (unless you're going somewhere that would require you to dress up, of course... and I probably wouldn't think much of someone overdressing, actually). It doesn't matter much to me what my date wears as long as she doesn't look like she rolled out of bed or was trying to pick up someone at the club (I think some guys might go for the latter, though, lol)