To start, no, "En Route Flight Advisory Service" and "Flight Service Station" are not the same thing. They can never be the same thing because they are entirely separate categories of thing—you are comparing apples and oranges, or more like apples and apple farmers. EFAS is a service and an FSS is the entity which provides that service; it is like asking if "keeping the roads clean" and "the Department of Public Works" are the same thing.
Next, En Route Flight Advisory Service, commonly known as "Flight Watch," does not exist anymore. This service was primarily used to provide pilots with in-flight updates on hazardous weather along their route; with the advancement of weather radar (air-, ground-, and space-based) and better in-aircraft display options it was decided to terminate that service in 2014 (see also this NBAA article which incorrectly defines EFAS as Emergency Flight Advisory Service). "EFAS" is listed as a term in JO 7240.2E (effective July 2014) but is not defined in JO 7340.2F (effective October 2015).
121.5 (VHF) and 243.0 (UHF) are the "emergency" or "guard" frequencies. They are monitored by ATC facilities across the country, and possibly by FSS as well. In addition, all pilots are required by FDC NOTAM 4/4386 (PilotWeb, PDF) to maintain a "listening watch" on these frequencies, if capable. You should use Guard to establish communications in the absence of any other clearly available communications channel, for example if you are a VFR pilot not receiving ATC services and you have an emergency, or if the ATC frequency you had been using becomes blocked by a stuck microphone.
Flight Service frequencies are described at many places around the Internet, for example this AOPA article (which is from 2008 and references EFAS and TWEBS which don't exist anymore).
I'm not entirely certain how Flight Service would be able to help you determine your location. It is technologically possible to triangulate the source of a radio broadcast, and some ATC radar displays in Europe apparently do this to highlight the data block of the aircraft currently transmitting—but ATC in the United States does not have this capability, and if ATC doesn't have it I imagine Flight Service doesn't either. Indeed, it's usually the opposite: because Flight Service frequencies are duplicated across the country (in order to provide constant coverage) you must tell them which of their receivers you are trying to engage with and which of their transmitters you are listening on! Not the most helpful thing if you don't know where you are yourself.
If you are truly lost—you have no iPad with a GPS moving map on it, you are above an overcast layer and cannot use pilotage, and you failed to properly pre-plan your flight so you have no idea of which ATC frequency is available in your location—you can call ATC on Guard, something like this:
You: Center, Skyhawk 12345, on Guard, I am lost and require location assistance.
ATC: Skyhawk 12345, Podunk Approach, ident. [you do]
ATC: Skyhawk 345, radar contact three-eight miles north-northwest of Podunk VOR. Change to my frequency 121.9.
