I just found threads that interested me and threw in my own two cents. Also-- if you don't feel much like holding a conversation, there's always the User Run Games forum where you don't have to worry about spamming much and things like the What Are You Listening To? thread in the Entertainment forum where you can post daily or periodically or whatever tickles your fancy.

Still in the process of doing that right now xD.

In addition to what mentioned, check out these forums and subforums: Achievements Games Avatar Contests & Games Small Talk

I think it was during the first month or two after I had resumed playing after the previous five years of sporadic activity. I hate having free time.
I honestly have no idea how I managed to get as many posts as I have :o I feel like I never join in on the forums at all, but I guess they just add up over time. I think site events help a lot as well :)
Most of my posts are from events where you claim the user above to dance or trick-or-treat or what have you. I've been trying to post the results of my dailies in the game forum lately too. I hate doing spammy stuff but I just don't find myself inclined to talk a lot and I want those forum points and posting achievements. I'm trying harder to be active in a more substantive manner though. Hence, for example, this post. Hi. XD
I got to my first 100 like 9 years ago, when the forums were more active and I was way more annoying.
I did a lot of interacting/role play during the first masquerade ball, it was a lot of fun since a lot of us who ended up talking with each other didn't really know one another until we struck up a small role play about finding dance partners.
Lair Bears Dance Mix! (play it on shuffle)
Man, I don't even know if I made it to a 100... lol. Events make me post like mad such as flowers, zombie bites, dancing, tralalala. :)
I tried chatting and making friends, but it's hard. :( So, I mainly stick to posting during event time. Can't wait to trick or treat!
mostly on the HA boards. the user run games are fun to pass the time :)
also during site events like the dance or kiss events etc
<( )
Probably by roleplaying. I used to do that a lot. Kind of miss it, but can't seem to get back into it.
Rii can draw! Check out her chibi art shop or her insta!
How about a webcomic about villains stumbling around redemption arcs? Drop by:

I didn't know this was something people cared about. I just tried to give advice, chatted along on plot boards, hung out on the usergames forum.
Avatar contests and games is a good one. Most of them involve a one-word comment on the person who posted before you.
The forums used to be more active when I started, so it was maybe a little easier for me. But hitting up any forums about a holiday seem to help?