Chatboxes and the like don't always take away from site activity. This is a myth that seems to crop up time and again on webmaster sites.
The truth is that things are more complicated than that.
In some cases they can diminish site activity. But in those cases they will only have negatively affected activity because people didn't care much about using the site to begin with.
In other cases chatboxes can increase activity. For example, roleplay sites they can be extremely useful for planning out ideas and stories to write on site. In these cases people are aware that the chatbox is to supplement the site, not supplant it.
On my own site I have a chatbox through chatango. It's an ok set up... It's got auto-refresh built right in (unlike cbox.ws which you need to pay for premium to get auto-refresh... which is a scam). I do like chatango's personal message system (so users can direct message each other without having to give out email addresses or anything like that). It also has a mobile app too so users can connect and chat on their smartphone.
As for how much use it gets... it varies since some days we're busy so we keep missing each other. Or we don't have a lot of time to chat. But then at times we'll chat a fair bit. (Especially when we're trying to set up a time to play SSB together.)
I'm with VirusZero here, but maybe it depends on the community itself. When my forum was still around, we had chat box from the first day but the forum still managed to reach 10K posts. People still talked at the forum and at chat box. If I remember correctly, somehow we discussed different topics at the chat box than what we had at the forum.
I think the key is to maintain good forum content; members will still come to post as long as it's interesting. And chat box does help to bring members of your community closer, so it's actually an interesting device.