Count users who complete events in the required order.

-- Build one row per member at each stage.
 WITH views AS(
  SELECT member_id,MIN(event_at) AS viewed_at
  FROM social_events
  WHERE event_type='view'
  GROUP BY member_id
), signups AS(
  SELECT v.member_id,v.viewed_at,MIN(e.event_at) AS signed_at
  FROM views v
  LEFT JOIN social_events e
    ON e.member_id=v.member_id
    AND e.event_type='signup'
    AND e.event_at>=v.viewed_at
  GROUP BY v.member_id,v.viewed_at
), funnel AS(
  SELECT
    s.member_id,
    s.viewed_at,
    s.signed_at,
    MIN(e.event_at) AS purchased_at
  FROM signups s
  LEFT JOIN social_events e
    ON e.member_id=s.member_id
    AND e.event_type='purchase'
    AND e.event_at>=s.signed_at
  GROUP BY s.member_id,s.viewed_at,s.signed_at
)
SELECT
  COUNT(*) AS viewed,
  COUNT(signed_at) AS signed_up,
  COUNT(purchased_at) AS purchased
FROM funnel;