DB Schema - Notification Service

I'm a little stuck with the database schema for an application I'm building and was wondering if anyone could give some advice?

Basically, I want to allow users to subscribe to certain events using a variety of services. For example..

When Event A is fired, it would send notifications via all methods that were subscribed. This might be 1 email address, 2 emails addresses, a Slack channel, a POST request to a URL, A tweet to Twitter or all of them.

An event is only ever run once. But, if this could be structured in such a way it supports an event running multiple times that would be even better.

I was thinking something like this:

| --------------------------------------------|
| Subscriptions                               |
| --------------------------------------------|
| id          | Primary Key                   |
| event_id    | ID of event subscribed to     |
| sub_id      | Object ID                     |
| sub_type    | Polymorphic Type              |
| --------------------------------------------|

| --------------------------------------------|
| Email                                       |
| --------------------------------------------|
| id          | Primary Key                   |
| address     | Email Address                 |
| --------------------------------------------|

| --------------------------------------------|
| Webhooks                                    |
| --------------------------------------------|
| id          | Primary Key                   |
| type        | Post or Get request           |
| url         | URL to trigger                |
| --------------------------------------------|

| --------------------------------------------|
| Twitter                                     |
| --------------------------------------------|
| id          | Primary key                   |
| auth_id     | Foreign ID for Auth table     |
| type        | DM or Tweet.                  |
| --------------------------------------------|