diff --git a/lib/pleroma/user.ex b/lib/pleroma/user.ex
index e422b59f1..1dde65335 100644
--- a/lib/pleroma/user.ex
+++ b/lib/pleroma/user.ex
@@ -146,6 +146,7 @@ defmodule Pleroma.User do
     field(:inbox, :string)
     field(:shared_inbox, :string)
     field(:accepts_chat_messages, :boolean, default: nil)
+    field(:last_active_at, :naive_datetime)
 
     embeds_one(
       :notification_settings,
@@ -2444,4 +2445,18 @@ def sanitize_html(%User{} = user, filter) do
   def get_host(%User{ap_id: ap_id} = _user) do
     URI.parse(ap_id).host
   end
+
+  def update_last_active_at(user) do
+    user
+    |> cast(%{last_active_at: NaiveDateTime.utc_now()}, [:last_active_at])
+    |> update_and_set_cache()
+  end
+
+  def active_user_count(weeks \\ 4) do
+    active_after = Timex.shift(NaiveDateTime.utc_now(), weeks: -weeks)
+
+    __MODULE__
+    |> where([u], u.last_active_at >= ^active_after)
+    |> Repo.aggregate(:count)
+  end
 end
diff --git a/lib/pleroma/web/mastodon_api/views/instance_view.ex b/lib/pleroma/web/mastodon_api/views/instance_view.ex
index 1edbdbe11..73205fb6d 100644
--- a/lib/pleroma/web/mastodon_api/views/instance_view.ex
+++ b/lib/pleroma/web/mastodon_api/views/instance_view.ex
@@ -45,6 +45,7 @@ def render("show.json", _) do
           fields_limits: fields_limits(),
           post_formats: Config.get([:instance, :allowed_post_formats])
         },
+        stats: %{mau: Pleroma.User.active_user_count()},
         vapid_public_key: Keyword.get(Pleroma.Web.Push.vapid_config(), :public_key)
       }
     }
diff --git a/lib/pleroma/web/plugs/user_tracking_plug.ex b/lib/pleroma/web/plugs/user_tracking_plug.ex
new file mode 100644
index 000000000..c9a988f00
--- /dev/null
+++ b/lib/pleroma/web/plugs/user_tracking_plug.ex
@@ -0,0 +1,30 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2021 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.Plugs.UserTrackingPlug do
+  alias Pleroma.User
+
+  import Plug.Conn, only: [assign: 3]
+
+  @update_interval :timer.hours(24)
+
+  def init(opts), do: opts
+
+  def call(%{assigns: %{user: %User{id: id} = user}} = conn, _) when not is_nil(id) do
+    with true <- needs_update?(user),
+         {:ok, user} <- User.update_last_active_at(user) do
+      assign(conn, :user, user)
+    else
+      _ -> conn
+    end
+  end
+
+  def call(conn, _), do: conn
+
+  defp needs_update?(%User{last_active_at: nil}), do: true
+
+  defp needs_update?(%User{last_active_at: last_active_at}) do
+    NaiveDateTime.diff(NaiveDateTime.utc_now(), last_active_at, :millisecond) >= @update_interval
+  end
+end
diff --git a/lib/pleroma/web/router.ex b/lib/pleroma/web/router.ex
index a9e332fa1..7521f5dc3 100644
--- a/lib/pleroma/web/router.ex
+++ b/lib/pleroma/web/router.ex
@@ -56,6 +56,7 @@ defmodule Pleroma.Web.Router do
     plug(Pleroma.Web.Plugs.UserEnabledPlug)
     plug(Pleroma.Web.Plugs.SetUserSessionIdPlug)
     plug(Pleroma.Web.Plugs.EnsureUserTokenAssignsPlug)
+    plug(Pleroma.Web.Plugs.UserTrackingPlug)
   end
 
   pipeline :base_api do
diff --git a/priv/repo/migrations/20210122151424_add_last_active_at_to_users.exs b/priv/repo/migrations/20210122151424_add_last_active_at_to_users.exs
new file mode 100644
index 000000000..9671e495b
--- /dev/null
+++ b/priv/repo/migrations/20210122151424_add_last_active_at_to_users.exs
@@ -0,0 +1,11 @@
+defmodule Pleroma.Repo.Migrations.AddLastActiveAtToUsers do
+  use Ecto.Migration
+
+  def change do
+    alter table(:users) do
+      add(:last_active_at, :naive_datetime)
+    end
+
+    create_if_not_exists(index(:users, [:last_active_at]))
+  end
+end
diff --git a/test/pleroma/web/mastodon_api/controllers/status_controller_test.exs b/test/pleroma/web/mastodon_api/controllers/status_controller_test.exs
index a647cd57f..3c73eb514 100644
--- a/test/pleroma/web/mastodon_api/controllers/status_controller_test.exs
+++ b/test/pleroma/web/mastodon_api/controllers/status_controller_test.exs
@@ -263,6 +263,7 @@ test "posting a fake status", %{conn: conn} do
 
       fake_conn =
         conn
+        |> assign(:user, refresh_record(conn.assigns.user))
         |> put_req_header("content-type", "application/json")
         |> post("/api/v1/statuses", %{
           "status" =>
diff --git a/test/pleroma/web/pleroma_api/controllers/conversation_controller_test.exs b/test/pleroma/web/pleroma_api/controllers/conversation_controller_test.exs
index 98a23aaaa..54f2c5a58 100644
--- a/test/pleroma/web/pleroma_api/controllers/conversation_controller_test.exs
+++ b/test/pleroma/web/pleroma_api/controllers/conversation_controller_test.exs
@@ -104,7 +104,7 @@ test "PATCH /api/v1/pleroma/conversations/:id" do
     [participation] = Participation.for_user(user)
     participation = Repo.preload(participation, :recipients)
 
-    assert user in participation.recipients
+    assert refresh_record(user) in participation.recipients
     assert other_user in participation.recipients
   end