Making screencast:
$ ffmpeg -f x11grab -s 1280x760 -r 25 -i :0.0+3840+0 -f alsa -ac 2 -i pulse -sameq video.mpg

Transcoding to webm:
$ ffmpeg -i video.mpg -s 1280x760 -vpre libvpx-720p -b 3900k -pass 1 -an -f webm -y video.webm

Output:
ffmpeg version 0.8.10-4:0.8.10-0ubuntu0.12.04.1, Copyright (c) 2000-2013 the Libav developers
  built on Feb  6 2014 20:56:59 with gcc 4.6.3
*** THIS PROGRAM IS DEPRECATED ***
This program is only provided for compatibility and will be removed in a future release. Please use avconv instead.
[mpeg @ 0x8807a0] max_analyze_duration reached
Input #0, mpeg, from 'dubna-cookies.mpg':
  Duration: 00:01:09.80, start: 1.000000, bitrate: 4105 kb/s
    Stream #0.0[0x1e0]: Video: mpeg1video, yuv420p, 1280x760 [PAR 1:1 DAR 32:19], 104857 kb/s, 25 fps, 25 tbr, 90k tbn, 25 tbc
    Stream #0.1[0x1c0]: Audio: mp2, 48000 Hz, stereo, s16, 128 kb/s
[buffer @ 0x883220] w:1280 h:760 pixfmt:yuv420p
[scale @ 0x883600] w:1280 h:760 fmt:yuv420p -> w:1280 h:720 fmt:yuv420p flags:0x4
[libvpx @ 0x884b00] v1.0.0
Output #0, webm, to 'dubna-cookies.webmc':
  Metadata:
    encoder         : Lavf53.21.1
    Stream #0.0: Video: libvpx, yuv420p, 1280x720 [PAR 18:19 DAR 32:19], q=11-51, pass 1, 3900 kb/s, 1k tbn, 25 tbc
Stream mapping:
  Stream #0.0 -> #0.0
Press ctrl-c to stop encoding
frame= 1747 fps= 75 q=0.0 Lsize=       0kB time=10000000000.00 bitrate=   0.0kbits/s
video:0kB audio:0kB global headers:0kB muxing overhead inf%




The content of the /usr/share/ffmpeg/libvpx-720p.ffpreset:
vcodec=libvpx
g=120
rc_lookahead=16
level=216
profile=0
qmax=42
qmin=10
vb=2M
#ignored unless using -pass 2
maxrate=24M
minrate=100k