You have good answers to your primary question and I believe the best take-away is that you now know Trulia IS NOT an MLS. Trulia is a "data syndicator".
The best move you can make now is to team with a RealtorÂ® in the target area you are searching so you have access to TRUE MLS information (and I highly recommend an automated search, which your RealtorÂ® can provide).
If you plan on "going it alone" please review the following post:
MLS Data Accuracy â€“ Where to search if youâ€™re â€œwithout RealtorÂ®â€
The best source of current listings is your realtor.
Trulia, depending on how the listings are set up for web exposure, may pick up the listings when they are brand new especially If the listings are associated with brokerages that have an arrangement with Trulia,
Sometimes, however, some agents may forget to update the status of their listings which may continue to show as active even if they are already pending.
Your best bet should you want to see only active listings (and to be notified of listings that come back to the market, or change prices) is to engage a realtor who can set it up so that you are automatically notified of such occurrence. Your agent will base the search on the MLS which is the realtors' primary service when posting listings.
What you see on Trulia could be a varierty of listings as well as non-listings (such as those distressed properties that receive notices of default or notices of trustee sale which are on public records, and provided via RealtyTrac).