GoQuest is mainstream, I guess people find it in their app store but you can also play in the browser http://wars.fm/. There are often hundreds of people playing live games and almost exclusively 9x9 and 13x13. Not sure what there is to talk about. You can search people by username but there isn't any interaction except playing and the time controls are probably too fast for many beginner, but that and the informal rating/ranking system probable makes it attractive for people who are affright to embarrass themselves.

It is also great when you don't feel like you have time for longer games.

Since I ranted I figure I owe an update. The Go Quest server slowly but surely got more and more consistent in match making. However, it took a bunch of games. I think it took about 80 games to reach my current "rank" (2k) and about 20 more games to settle the "elo" (the other number in the hundreds). If you are even stronger then I assume that it will take you even longer to get settled. If you are a beginner, it might not take too long to get settled.

By the way, I was under the impression that Go Quest ranks were completely trivial but that is not the case. I'm not sure if it was different before but there is actually a minimum elo requirement and a minimum number of win requirement to rank up. For example, I ranked up to 2k at 1500 but I will need to be 1600 to rank up again. Maybe the requirements were different in the past though, because there seem to be several players at my elo (and skill level) but who are ranked 2dan+, which seems unlikely.

I think GoQuest uses the basic ELO ratings because the help file says that and the rating updates are consistent with that at first glance. ELO rating is like this:

new rating := old rating + K * ( outcome - expected outcome )
expected outcome := 1 / ( 1 + 10 ** ( rating difference / 400 ) )
rating difference := opponents rating - old rating

The variable K depends on rating in Chess ELO rating, it is 32 until 2000, then 24 and finally 16 for over 2400 rating.

Fairly simple, but GoQuest seeds new players in with a high rating (probably 1500 because this number is mentioned in the help file). They say in the help file that they want to motivate people by having the rating increase during the early games instead of dropping down from 1500. To this purpose the rating for the first 40 games is displayed as if it was increasing from a lower rating.

The rank is like you said based on reaching certain rating and having a winning streak so you can miss out on a promotion if your rating drops without achieving the winning streak but then it has been the case that the winning streak can start before you reach the rating. Usually when the rating increases it is due to a winning streak, I don't think requiring such winning streaks changes much but it must have been the intention that random fluctuation in rating would be less likely to result in promotion.

There are many players with high ranks that then fall back down. The requirements in 13x13 are like this, checking recent opponents:
8 dan: win 10 in a row, rating >= 2700
7 dan: win 8 in a row, rating >= 2350
6 dan: win 7 in a row, rating >= 2200
5 dan: win 6 in a row, rating >= 2100
4 dan: win 5 in a row, rating >= 2000
3 dan: (probably) win 4 or 5 in a row, rating >= 1900
2 dan: win 4 in a row, rating >= 1800

I played players 2-300 points below their rank requirement, usually people will reach a peak rank and then fall back a little bit. There are also sometimes players that are winning almost every game but their rank and rating is still very low.

I play 9x9 on it a fair bit. I found I actually got to my rank and rating reasonably quickly (6 dan, although my rank is typically between 2100 and 2200.)

My main annoyance is that it sometimes doesn't recognise seki correctly. There are a few losses on my record that are actually wins.

Really? i didnt have any problems with seki so far. Could you post an example that was recognized correctly?

I think the games may be lost to posterity. (I seem to get my last records and no more.) It is typically sekis with unusual features, for example where one player has an eye and the other doesn't.

Has anyone noticed a change in the 9x9 match making lately or is it just me? For a few months I've been around 1500-1600 rating myself and typically matched against 1250-1800 rated players. I sometimes get a handicap stone and sometimes give one.

But lately I've been getting a few really highly rated players matched against me. And sometimes without handicap stones. I played: 1896 (handicap stone at 2-2), 1907 (no handicap), and a red-colored 2201 (no handicap). I've not been doing particularly well lately myself (from ~1600 down to ~1530) so that doesn't explain it. I follow Yanagisawa Satoshi 6-dan on YouTube. He comments some 13x13 Go Quest games and a few 9x9 games. His account rating is 2100 on 9x9... Take it easy on me, I'm waiting for boiling water or the microwave.

By the way, does Go Quest ever do 0 komi? I noticed some weird counting lately that might be attributed to that, but I don't know how to check the komi setting (if there is one).