chimein

Brand-hosted forums FACILITATES communication all the way across the customer life cycle. A website with visitor feedback is much more trusted than one without it. ChimeIn adds a dialog layer to your page without interfering with your content.
Fetaures: – simplest to add to site – as a separate layer, you have no legal liability on user generated content – ChimeIn does basic moderation of reported posts 
– ChimeIn user notifications create a social hook that makes your visitors return
As a free user, you will get limited moderation rights. By subscribing for ChimeIn you will get: – highlighted posts – you can highlight the most constructive visitor posts – insights and analytics on visitor behavior, content and sentiment
Don’t take the dialog to social media. It’s three times more likely that someone interested in you or your brand goes to your website, not your Facebook page. Don’t send them away, let them converse on your site.

ChimeIn Sidewall

Very easy setup, immediate dialog on the site.

Very simple, very efficient

Add Conversation to Your Site. ChimeIn adds a dialog layer to your page without interfering with your content.

Yannick Lefebvre

The purpose of this plugin is to allow users to create one or more modal dialog(s) / pop-up window(s) that will appear when a user visits their site. The number of times that these can load is configurable. They can load content from external sites or custom HTML code into the dialog. This plugin can be used to invite people to register to a newsletter, respond to a survey, or simply welcome come to a site upon their first visit.
You can try it out in a temporary copy of WordPress <a href="https://demo.tastewp.com/modal-dialog" rel="nofollow ugc">here</a>.

Modal Dialog

The others either didn't work or caused massive JavaScript problems, breaking other plugins.

This plugin worked the best.

Worked better than the others

The purpose of this plugin is to allow users to create one or more modal dialog(s) / pop-up window(s) that will appear when a user visits their site.

ChimeIn Sidewall vs Modal Dialog